Add welcome text for the queries panel

[robertbrignull]

This gives some default text to display in the queries panel where there aren't any queries in the workspace. Suggestions welcome for what to put as the text, and we can always change the text later on.

This shows whenever QueryTreeDataProvider returns no children for the root node, with no extra configuration needed. One complication is it also shows while the CodeQL extension is loading, so ideally the text shouldn't be confusing in this situation.

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]

This might not be visible until #2447 is merged because that PR changes the behaviour to not include workspace folders which don't include any queries, as otherwise it'll always show an entry for the workspace folder, even if that directory has no children.