Add package `core:flags`

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diff --git a/core/flags/doc.odin b/core/flags/doc.odin
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+/*
+package flags implements a command-line argument parser.
+
+It works by using Odin's run-time type information to determine where and how
+to store data on a struct provided by the program. Type conversion is handled
+automatically and errors are reported with useful messages.
+
+
+Command-Line Syntax:
+
+Arguments are treated differently depending on how they're formatted.
+The format is similar to the Odin binary's way of handling compiler flags.
+
+```
+type                  handling
+------------          ------------------------
+<positional>          depends on struct layout
+-<flag>               set a bool true
+-<flag:option>        set flag to option
+-<flag=option>        set flag to option, alternative syntax
+-<map>:<key>=<value>  set map[key] to value
+```
+
+
+Struct Tags:
+
+Users of the `core:encoding/json` package may be familiar with using tags to
+annotate struct metadata. The same technique is used here to annotate where
+arguments should go and which are required.
+
+Under the `args` tag, there are the following subtags:
+
+- `name=S`: set `S` as the flag's name.
+- `pos=N`: place positional argument `N` into this flag.
+- `hidden`: hide this flag from the usage documentation.
+- `required`: cause verification to fail if this argument is not set.
+- `variadic`: take all remaining arguments when set, UNIX-style only.
+- `file`: for `os.Handle` types, file open mode.
+- `perms`: for `os.Handle` types, file open permissions.
+- `indistinct`: allow the setting of distinct types by their base type.
+
+`required` may be given a range specifier in the following formats:
+```
+min
+<max
+min<max
+```
+
+`max` is not inclusive in this range, as noted by the less-than `<` sign, so if
+you want to require 3 and only 3 arguments in a dynamic array, you would
+specify `required=3<4`.
+
+
+`variadic` may be given a number (`variadic=N`) above 1 to limit how many extra
+arguments it consumes.
+
+
+`file` determines the file open mode for an `os.Handle`.
+It accepts a string of flags that can be mixed together:
+- r: read
+- w: write
+- c: create, create the file if it doesn't exist
+- a: append, add any new writes to the end of the file
+- t: truncate, erase the file on open
+
+
+`perms` determines the file open permissions for an `os.Handle`.
+
+The permissions are represented by three numbers in octal format. The first
+number is the owner, the second is the group, and the third is other. Read is
+represented by 4, write by 2, and execute by 1.
+
+These numbers are added together to get combined permissions. For example, 644
+represents read/write for the owner, read for the group, and read for other.
+
+Note that this may only have effect on UNIX-like platforms. By default, `perms`
+is set to 444 when only reading and 644 when writing.
+
+
+`indistinct` tells the parser that it's okay to treat distinct types as their
+underlying base type. Normally, the parser will hand those types off to the
+custom type setter (more about that later) if one is available, if it doesn't
+know how to handle the type.
+
+
+Usage Tag:
+
+There is also the `usage` tag, which is a plain string to be printed alongside
+the flag in the usage output. If `usage` contains a newline, it will be
+properly aligned when printed.
+
+All surrounding whitespace is trimmed when formatting with multiple lines.
+
+
+Supported Flag Data Types:
+
+- all booleans
+- all integers
+- all floats
+- all enums
+- all complex numbers
+- all quaternions
+- all bit_sets
+- `string` and `cstring`
+- `rune`
+- `os.Handle`
+- `time.Time`
+- `datetime.DateTime`
+- `net.Host_Or_Endpoint`,
+- additional custom types, see Custom Types below
+- `dynamic` arrays with element types of the above
+- `map[string]`s or `map[cstring]`s with value types of the above
+
+
+Validation:
+
+The parser will ensure `required` arguments are set, if no errors occurred
+during parsing. This is on by default.
+
+Additionally, you may call `register_flag_checker` to set your own argument
+validation procedure that will be called after the default checker.
+
+
+Strict:
+
+The parser will return on the first error and stop parsing. This is on by
+default. Otherwise, all arguments that can be parsed, will be, and only the
+last error is returned.
+
+
+Error Messages:
+
+All error message strings are allocated using the context's `temp_allocator`,
+so if you need them to persist, make sure to clone the underlying `message`.
+
+
+Help:
+
+By default, `-h` and `-help` are reserved flags which raise their own error
+type when set, allowing the program to handle the request differently from
+other errors.
+
+
+Custom Types:
+
+You may specify your own type setter for program-specific structs and other
+named types. Call `register_type_setter` with an appropriate proc before
+calling any of the parsing procs.
+
+A compliant `Custom_Type_Setter` must return three values:
+- an error message if one occurred,
+- a boolean indicating if the proc handles the type, and
+- an `Allocator_Error` if any occurred.
+
+If the setter does not handle the type, simply return without setting any of
+the values.
+
+
+UNIX-style:
+
+This package also supports parsing arguments in a limited flavor of UNIX.
+Odin and UNIX style are mutually exclusive, and which one to be used is chosen
+at parse time.
+
+```
+--flag
+--flag=argument
+--flag argument
+--flag argument repeating-argument
+```
+
+`-flag` may also be substituted for `--flag`.
+
+Do note that map flags are not currently supported in this parsing style.
+
+
+Example:
+
+A complete example is given in the `example` subdirectory.
+*/
+package flags