Show selected item in new DB panel

[shati-patel]

Adds a FileDecorationProvider which displays a ✔ label next to the selected item in the new DB panel. E.g.

It currently doesn't have any clicky UI actions, but that will come in a follow-up issue!

Note: I'm not sure if/how to test this "file decoration". I'm wondering whether to wait until we have UI actions too, but I'm open to other suggestions about tests!

Checklist

N/A - internal only 🦑

• 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]

LGTM 🎉 , just a comment around where the logic goes.

I don't think we need to write any further tests - I already covered some of this logic in the previous PR.