Add language filter panel.

[starcke]

This adds a language selection panel to the extension. This hooks into the filtering, so that selecting an item in the panel filters to that language.

This is the first time I added a new panel so I am quite unsure whether it is done in the right way, feedback is very welcome.

Few notes:

• Is it enough to feature flag in package.json so that the panel is never shown or do we need to do it in extension.ts as well?
• I could also have read the languageContext in the getTreeItem method instead of passing in selected. However I wasnt sure how often it would call getTreeItem and it seemed a bit nicer to just set it on construction, but I am happy to change it.

Checklist

• CHANGELOG.md has been updated to incorporate all user visible changes made by this pull request.
• Issues have been created for any UI or other user-facing changes made by this pull request.
• [Maintainers only] If this pull request makes user-facing changes that require documentation changes, open a corresponding docs pull request in the github/codeql repo and add the ready-for-doc-review label there.

[koesie10]

Is it enough to feature flag in package.json so that the panel is never shown or do we need to do it in extension.ts as well?

This should be enough, the extension.ts doesn't really have any way of controlling whether a panel is shown or not.

I could also have read the languageContext in the getTreeItem method instead of passing in selected. However I wasnt sure how often it would call getTreeItem and it seemed a bit nicer to just set it on construction, but I am happy to change it.

I think this is fine, another alternative would be to construct the tree items in getChildren, but I don't think that has any advantages over this approach either.

[starcke]

Addressed comments.

[koesie10]

Just some small stylistic comments, but the code in general looks good.