fix: fix atx heading and make regex safe

[UziTech]

Marked version: v1.2.5

Description

Make heading regex safe from redos and pass all atx heading spec tests.

fixes CommonMark spec test 49

possible breaking change: When using the block.heading rule the text provided in capture group 2 will not be trimmed of whitespace.

Since this is only a breaking change to the capture groups this doesn't require a major release but the previous statement should be copied to the release notes.

Contributor

• Test(s) exist to ensure functionality and minimize regression

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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[github-actions]

🎉 This PR is included in version 1.2.6 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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