Add stubs of commands for multi-query support

[robertbrignull]

This PR adds stubs of commands for codeQL.runVariantAnalysisContextExplorer and codeQL.runVariantAnalysisPublishedPack. Both of these are currently behind the canary flag and a new codeQL.variantAnalysis.multiQuery. We're not trying to fully implement these commands in this PR, but just add the boilerplate that we can extend later.

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.

[charisk]

Thanks, LGTM!