Introduce separate tree item types in the methods usage panel

[koesie10]

This creates new tree item types for methods and usages such that these can contain references to their parent and children. This allows us to easily find the parent of a usage and to find the children of a method. This removes an expensive find call in getParent.

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]

Works for me when tested, and the code looks good. I think this is a good approach and very similar to what I tried to implement previously.

I also think renaming resolveCanonicalUsage to resolveUsageTreeViewItem is a good change. Unfortunately we can't remove this method unless the internal tree view data/type is the exact same as the data outside of the tree view, and that won't be the case now. We still do an expensive lookup in this method but I think this is an ok tradeoff and we call this a lot less frequently than we call getParent. It's also something we can look at improving again in the future if it's an issue.