replacement of "." in headings for slugs

[dkuegler]

What version of myst-parser are you using?

2.0.0

What version dependencies are you using?

Sphinx 7.2.6

What operating system are you using?

Linux

Describe the Bug

Anchors are generated differently in Myst parser and GitHub markdown parsers. Specifically,
# Ubuntu 20.04 gets the anchor ubuntu-2004 on GitHub, but ubuntu-20-04 by Myst parser

Expected Behavior

Same anchor as GitHub.

To Reproduce

See above

[chrisjsewell]

Same anchor as GitHub.

Well thats the difficulty; github doesn't have good specifications for its syntaxes 😒

But actually I have written an entire slugger in rust recently 😉 : https://github.com/markdown-it-rust/markdown-it-plugins.rs/tree/main/crates/github_slugger