feat: add glimmer js/ts markdown language injects

[IgnaceMaes]

Adds support for gjs and gts embeds in Markdown.

Based on https://github.com/mjbvz/vscode-fenced-code-block-grammar-injection-example

Open issue: <template> tag is not picked up correctly. I did some fiddling around but haven't pinpointed the exact issue. Opening the PR already just in case anyone else sees the fix.

[IgnaceMaes]

I rebased to master and with the way the merging of syntaxes has changed, I believe it fixed the issue of the <template> syntax not getting embedded into md properly.

This is the same example now:

And trying something a bit more challenging:

[lifeart]

@IgnaceMaes, thank you! Great work!