Add code lens for "Open referenced file"

[shati-patel]

Makes the "Open referenced file" command more discoverable by adding a code lens:

Click that prompt to jump to the referenced QL file 😎 See internal issue for more details! 🔍

Notes:

• I put this in the local-queries folder, which seemed reasonable 😅 Happy to be pointed to a more suitable location!
• I couldn't think of a useful way to test this, since it just calls out to the existing codeQL.openReferencedFile command...

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.

[robertbrignull]

You may want to wait for a review from @github/codeql-vscode-reviewers, but the code looks good to me and I tried testing it and the codelens worked and was intuitive.

[aeisenberg]

That's a great idea. Thanks for implementing!