Tree widget: Update to iTwinUI v5

[grigasp]

Now that the first 5.0-alpha of iTwinUI is out, we should start the work of updating the tree widget to use the new APIs. The goals:

• Identify missing pieces and bugs as early as possible and let iTwinUI team know about that.
• Do as much work as possible before the full iTwinUI 5.0 release so we can do our own release quickly afterwards.

This is likely take multiple PRs and span a long period of time. And we don't want to release until we consume the full 5.0. So I suggest setting up a protected branch and doing all the work there.

Related: iTwin/presentation#832