Apply language context to queries panel

[starcke]

This adds language filtering to the queries panel. Adding the filtering was trivial, however there were a few other tweaks I had to make.

1. Previously we made onDidChangeQueries the same as onDidChangePathData. However if we integrated with that it would require a full rescan of the file system to apply a filter which felt quite slow for me. Instead we create a specific emitter for onDidChangeQueries which is triggered by either the filesystem changing or the filter changing.
2. Initially my tests failed when trying to dispose this.languageContext.onLanguageContextChanged. This was because the MockApp created a MockEventEmitter which returned {} as disposable. However {} is actually not disposable (because it lacks the method). So I changed it to return something that is actually disposable.

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.

[shati-patel]

This looks good to me! Works locally, and your explanations for onDidChangeQueries and the disposable make sense.