Add support for GitLab slugs to TOC (Closes #469)

[BeeeWall]

This removes the githubCompatibiity option, replacing it with slugifyMode. Unfortunately, while I originally tried to make it automatically set the latter based on the former, I couldn't figure out a way to actually "delete" the setting (setting it to undefined doesn't seem to work, though it says it should), and as such any changes to slugifyMode would always be overridden by githubCompatibility. So I had to settle for just removing it (unless someone else knows how to do that right?)

[BeeeWall]

Is there a reason the slugify tests don't call mdHeadingToPlainText before calling slugify? (I spent an embarrassingly long time trying to figure out why GitLab gave a different link from my code [well, my replication of their code], before I realized it was that they strip syntax first and the tests don't lol)

Also, I've updated .tsconfig to use ES2018, as the minimum version of VS Code you put should support it, and this allows replacing that couple-thousand-character punctuation regex that had been in there.

[BeeeWall]

Along the way, I also discovered GitLab has their own feature for Table of Contents (putting [[_TOC_]] will insert one when rendered in GitLab).

I'm not sure if that's relevant to this extension in any way or not, but it may be useful for anyone who stumbles across this, I suppose.

[yzhang-gh]

Thanks a lot!

This removes the githubCompatibiity option, replacing it with slugifyMode.

No worries. We can pop up a warning if people have enabled githubCompatibility. I'll do it later.

Is there a reason the slugify tests don't call mdHeadingToPlainText before calling slugify?

vscode mode doesn't do mdHeadingToPlainText so we need to put it inside slugify function.

Also, I've updated .tsconfig to use ES2018, as the minimum version of VS Code you put should support it, and this allows replacing that couple-thousand-character punctuation regex that had been in there.

Interesting! Nice change.

Along the way, I also discovered GitLab has their own feature for Table of Contents (putting [[_TOC_]] will insert one when rendered in GitLab).

We can mention this in the original issue. From technical perspective, it is doable microsoft/vscode#85682 (comment). But maybe we don't need it now.