Populate details panel with external API usages

[robertbrignull]

This PR adds ModelDetailsPanel and ModelDetailsDataProvider, and populates the tree view with data whenever new external API usages are calculated.

This isn't perfect, but everything is hidden behind a feature flag so I think this is a suitable step in the right direction. There are still lots of changes to make, which we'll do in future PRs. For example:

• Handling when there are multiple data extensions editor windows open at once.
• Making sure data is always updated when new external API usages are calculated.
• Styling of the details panel, with icons and better labels.
• Jumping to definition when clicking an item.

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.

[norascheuch]

🚀