feat: validate Literal[...] annotations as choices

[bhirsz]

Summary

Adds choice validation driven by typing.Literal[...] annotations.

Annotate an option (or argument) with Literal[...] to restrict it to a fixed
set of allowed values:

from typing import Literal
from confargs import ArgConfig, option

class Args(ArgConfig):
    console: Literal["verbose", "dotted", "quiet", "none"] = option(name="console", default="verbose")
    level: Literal[1, 2, 3] = option(name="level", default=1)
    langs: list[Literal["en", "pl"]] = option(name="langs", default=list)

Behaviour

• The incoming value is coerced to the members' type (so Literal[1, 2, 3]
  accepts the string "2" from the CLI and yields 2), then checked for
  membership.
• An out-of-set value raises OptionValueError listing the allowed choices.
• Works for scalars, list[Literal[...]], Literal[...] | None, method value
  parameters, positional arguments, and TOML/env sources.
• --help renders the allowed values as a {a,b,c} metavar.

Changes

• coercion.py: ValueType.choices, Literal detection in _analyse,
  _analyse_literal, and membership enforcement in coerce_value.
• help.py: choices metavar.
• README.md: "Restricting a value to a set of choices" section.
• tests/test_choices.py: 10 tests.

Refs #22

Part of the option-ergonomics stack: #23 (attribute typing) → #24 (default
factory) → this PR. Enables the PR #20 example to drop manual choice validation.