Add pytest.ini support for only_rerun

[LouisDeconinck]

Fixes #165.

Add the missing pytest.ini configuration for --only-rerun while preserving command-line precedence. The setting accepts one regex per line, matching the existing repeatable CLI option.

Included a regression test, configuration documentation, and a towncrier feature entry.

[LouisDeconinck]

Added coverage for multiple only_rerun entries.

[icemac]

Thanks for picking this up! Precedence (CLI beats ini), pyproject.toml support, -o only_rerun= and --help rendering all check out. A few remarks inline, mostly about the sibling option, tests and docs.

— Comment created by Claude

[LouisDeconinck]

All remarks addressed in 9e9a542:

• rerun_except is now a registered ini option (type="linelist"), mirroring only_rerun — the fallback in _get_rerun_filter_regex is now a plain getini(regex_name) for both names.
• Hoisted ONLY_RERUN_DESC / RERUN_EXCEPT_DESC shared by the CLI flags and addini, mentioning the one-per-line ini form.
• check_options now compiles each CLI/ini pattern and raises UsageError naming the option, so a committed [unclosed fails fast with a clear message instead of an INTERNALERROR traceback.
• Tests: CLI-overrides-ini precedence (asserts the R-marker lands on the ValueError test), marker-overrides-ini, ini only_rerun combined with --rerun-except exercising the four-way branch, rerun_except from ini, and invalid-regex cases for both names.
• Docs: README gained the ini forms in both filter sections plus the marker > CLI > ini order in Priority; configuration.rst documents rerun_except, the replace-not-accumulate behavior, and the marker-interaction caveat; mark.rst mentions the ini tier.

234 tests pass; ruff and sphinx-lint clean.