Unreliable minimap redraw

[mchakravarty]

Getting the minimap to redraw properly in case of changes affecting a larger area has proven difficult and OS-specific. It depends on timing and concurrency issues in the text system and I haven't been able to find a reliable way of tackling this problem.

At the moment, we have some invalidation/relayout kludges that vary somewhat across macOS, iOS, and visionOS — see also [Note Minimap Redraw Voodoo].

The easiest solution may be to use a custom view for the minimap instead of re-purposing {NS|UI}TextView. Given that we can (hopefully) reuse all other components of TextKit 2, this shouldn't be too difficult. See also the TextKit 2 video and example code from WWDC 2021.