Add support for custom href prefix in footnotes

[Copilot]

Summary

This PR adds support for customizing the href attribute prefix in footnote links via the env.footnoteHrefPrefix parameter. This addresses the feature request to make footnote hash support custom href (often prefix), similar to the pattern used in markdown-it-github-headings.

Changes Made

• Understand the current implementation and how it generates footnote IDs and hrefs
• Add support for custom href prefix through env.footnoteHrefPrefix
• Update render functions to use the custom prefix
• Add comprehensive tests for custom href prefix functionality
• Ensure backward compatibility (default behavior unchanged)
• Run lint, build and tests
• Update README with documentation and examples
• Run security checks
• Remove package-lock.json per review feedback

Core Implementation

• Added render_footnote_href_prefix() helper function that returns the custom prefix from env.footnoteHrefPrefix or defaults to #
• Updated render_footnote_ref() to use the custom href prefix for footnote references
• Updated render_footnote_anchor() to use the custom href prefix for back-references
• Registered the new helper function in the renderer rules

Use Cases

This feature enables:

• Linking footnotes to different pages (e.g., /docs/page#fn1)
• Custom URL structures for footnotes (e.g., https://example.com/docs#fn1)
• Combining with docId for both custom IDs and custom hrefs

Documentation

• Added new "Custom href prefix" section in README.md
• Included examples showing usage with and without docId

Tests

• Added test fixtures for custom href prefix usage
• Added test for combined docId and custom href prefix
• All 20 tests pass (16 existing + 4 new)
• 100% code coverage maintained

Backward Compatibility

• ✅ Maintains full backward compatibility - defaults to # when not specified
• ✅ Works seamlessly with existing env.docId feature
• ✅ All existing tests continue to pass

Original prompt

Please make footnote hash support custom href (often prefix).
Reference Flet/markdown-it-github-headings#18

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[ChenPi11]

Please remove package-lock.json.

[Copilot]

Removed in commit 9a7260c.

[ChenPi11]

@copilot Please solve the requested changes: "Please remove package-lock.json."

[Copilot]

@copilot Please solve the requested changes: "Please remove package-lock.json."

Done. Removed package-lock.json in commit 9a7260c.