Add support for GitLab slugger

[Kamik423]

Describe the feature you'd like to request

The GitLab slugger’s source is here, it would be great if MyST provided that function to use as a drop-in replacement for the GitHub one.

This would be helpful to me, since I want my links to work both in GitLab Markdown files and in MyST.

Describe the solution you'd like

No response

Describe alternatives you've considered

No response