✨ NEW: Support configurable front matter markers

[Gonghan-Princess]

Front matter delimiters are currently fixed to ---, preventing applications from using +++ for TOML-style metadata or ;;; for JSON-style metadata. Add a keyword-only marker option to front_matter_plugin, configured separately for each MarkdownIt parser.

For example, MarkdownIt().use(front_matter_plugin, marker="+") extracts content between +++ fences. The plugin continues to return raw metadata text; it does not decode TOML or JSON or add dependencies. The default - marker and existing YAML fixtures retain their behavior. Custom markers use the existing minimum-three and matching-closing-length rules; YAML's ... terminator is recognized only with the default marker.

Reject empty, multi-character, whitespace, and NUL markers when configuring the plugin. Update the API documentation with an example and add an Unreleased changelog entry.

Fixes #134.

Validation on Windows / Python 3.12.10:

• Initial regression run: 23 feature cases failed on the original API; nine existing front matter tests passed. An additional test reproduced the NUL normalization edge before adding its validation.
• Project tox -e py312: 535 passed (24 added cases).
• Project tox -e mypy, tox -e pre-commit, and tox -e docs-update: passed, including strict Sphinx -nW --keep-going.
• Executed the Python example extracted from the actual function docstring successfully.
• Tests cover token content/markup/maps, hidden rendering, TOML/JSON delimiters, empty metadata, short/invalid/unclosed/mismatched fences, YAML terminator text inside custom metadata, and independent parser configurations.
• Other Python versions and platforms remain for CI.

AI assistance: OpenAI Codex prepared the implementation, tests, documentation, and PR text. A separate Codex agent reviewed the patch; its NUL-marker finding was addressed and tested. No human review is claimed.