feat: add opt-in case- and hyphen-insensitive CLI option matching

[bhirsz]

Summary

Adds two opt-in, per-class attributes that relax long-option matching on the
command line:

• cli_case_insensitive — --VariableFile resolves to --variablefile
• cli_ignore_hyphens — --variable-file resolves to --variablefile

Both default to off, so existing behaviour is unchanged unless a tool opts
in. With both enabled, --variablefile, --variable-file, --VariableFile and
--VARIABLE-FILE all resolve to the same option, and a flag can be negated as
--no-statusrc, --nostatusrc or --No-StatusRc.

Configuration files stay exact

Leniency applies to the command line only. A [tool.<tool>] table must use
the option's declared name; its underscore/hyphen variants remain interchangeable
(pre-existing behaviour), but case is significant and arbitrary hyphenation is
not accepted. This keeps config files unambiguous and diff-friendly.

How it works

• A normalised fallback map (long_normalized_to_attr) is built next to the
  exact long_to_attr map. Lookups try the exact name first, then the
  normalised fallback when leniency is enabled (NameTable.long_attr).
• Normalisation lower-cases (when cli_case_insensitive) and strips hyphens
  (when cli_ignore_hyphens).
• Flag negation understands both the canonical --no-<name> form (with lenient
  base resolution) and, when hyphens are ignorable, the joined --no<name> form.
• If two options collide once normalised (e.g. --foo-bar vs --foobar), the
  shared fallback is dropped — both still work through their exact spellings.

Why

Requested for the Robot Framework CLI migration
(robotframework/robotframework#5773),
whose legacy parser accepted case- and hyphen-insensitive long options. Making
this a confargs feature (rather than pre-processing argv on the tool side) keeps
the tool integration thin and lets other tools opt in too.

Changes

• src/confargs/base.py — new cli_case_insensitive / cli_ignore_hyphens
  class attributes (documented).
• src/confargs/options.py — NameTable gains the normalised fallback map and
  long_attr / normalize_bare helpers; resolve_names builds the fallback.
• src/confargs/cli.py — long-option and negation resolution use lenient lookup.
• src/confargs/processor.py — passes the class toggles into resolve_names;
  eager scanning uses lenient lookup.
• tests/test_lenient_names.py — 17 new tests (spellings, negation forms,
  opt-in-only, single-toggle behaviour, normalised-collision fallback, config
  exactness, end-to-end).
• README.md / CHANGELOG.md — documented under "Options in depth" and an
  [Unreleased] Features entry.

Validation

uv run ruff check ., uv run ruff format --check ., uv run mypy and
uv run pytest -q (308 tests) all pass.