[GFM] Implement the tagfilter extension

[Leo (LouisLou2)]

Summary

Implement the GFM tagfilter extension for raw HTML rendering.

The extension replaces the leading < with < for the following tags, matched case-insensitively:

• title
• textarea
• style
• xmp
• iframe
• noembed
• noframes
• script
• plaintext

Both inline HTML and HTML blocks are covered. Other HTML tags and CommonMark rendering remain unchanged.

This also tightens the type 7 HTML block start condition: after a complete opening or closing tag, only whitespace may appear before the end of the line. The previous condition incorrectly classified lines such as <strong> <title> as HTML blocks.

Closes #211

Tests

Added coverage for:

• the official GFM tagfilter example
• inline and block HTML
• opening, closing, and self-closing tags
• attributes and case-insensitive matching
• allowed HTML tags
• similar tag names such as <scripture>
• unchanged CommonMark behavior

Existing GFM HTML block expectations were updated because GFMFlavourDescriptor enables GFM extensions globally.

Verification

• ./gradlew jvmTest
• ./gradlew jsNodeTest

Closes #ISSUE_NUMBER

[Ilia Permiashkin (p3rmiashkin)]

Leo (@LouisLou2) Hi! Can you please rebase your branch on the latest master and add a note to CHANGELOG.md?

[Leo (LouisLou2)]

Leo (Leo (@LouisLou2)) Hi! Can you please rebase your branch on the latest master and add a note to CHANGELOG.md?

Done! I've rebased the branch onto the latest master and added a changelog entry under Unreleased.