feat: add case-insensitive choice matching

[bhirsz]

Summary

Adds an ignore_case flag to option() and argument(). When set and the value is constrained by Literal[...] choices, the value is matched case-insensitively and normalised to the spelling declared in the Literal.

class Args(ArgConfig):
    # --colors on / On / ON  all yield "ON"
    colors: Literal["AUTO", "ON", "OFF", "ANSI"] = option(
        name="colors", default="AUTO", ignore_case=True
    )

• Unknown values are still rejected (invalid value 'purple'; choose from ...).
• A case-insensitive match is only honoured when unambiguous.
• Works for scalars, list[Literal[...]], method value parameters, positional argument()s, and values coming from TOML/env — the check lives in coerce_value.
• Default is unchanged (case-sensitive), so existing options are unaffected.

Motivation

Tools often accept case-insensitive enum-style values whose canonical form is a fixed spelling (e.g. Robot Framework's --consolecolors auto|on|off|ansi, stored canonically as uppercase). Until now, using Literal for those would reject the lowercase spellings users type, so such options had to fall back to plain str and lose validation. With ignore_case=True they can use Literal and gain shell completion of the allowed choices and early validation, while still accepting any casing and handing downstream code the canonical spelling.

Tests

• tests/test_choices.py: scalar normalisation, unknown-value rejection, list choices, arguments, TOML source, and a guard that the default stays case-sensitive.
• Full suite: 371 passed; mypy --strict clean; ruff lint + format clean.

Docs

• README "Restricting a value to a set of choices" gains an ignore_case example.
• CHANGELOG [Unreleased] Features entry.