fix: support GFM protocol autolinks

[evgenyponomarev]

Marked version: master

Markdown flavor: GitHub Flavored Markdown

Description

• Addresses the protocol-autolink subgroup of CommonMark spec gaps grouped by root cause, and a question about lookup tables #4050 (GFM 0.29 examples 633–635).
• Recognizes mailto: and xmpp: extended autolinks using the GFM email grammar.
• Supports one optional XMPP resource segment and stops before a second slash.
• Leaves the bare-email domain fix in fix: reject GFM email autolink when the domain ends in _ or - #4063 untouched.

AI disclosure: OpenAI Codex assisted with implementation and validation; I reviewed and own the contribution.

Contributor

• Tests exist to ensure functionality and minimize regression.
• This is existing GFM behavior, so no feature documentation is required.

Testing

• npm test
• npm run rules

Committer

In most cases, this should be a different person than the contributor.

• CI is green (no forced merge required).
• Squash and Merge PR following conventional commit guidelines.

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