feat(undotree plugin): adds undotree support and a sidecar concept

[obukhovaa]

Description

For those who can't imagine coding without undotree even in a chill mode.

The initial idea was to somehow fit it into the existing zen-mode's plugin concept, but I've quickly realised that to make it happen a new "sidecar-plugin" concept has to emerge, it effectively allows any other plugin to attach its window(s) to the main zen-view to the left or right. Since the idea of zen-mode is to not overload user with the information, only one plugin is allowed to register a sidecar.
There's a few minor places where Im not completely happy with the solution, to be specific:

• It turns out it is quite an unpleasant experience to deal with the navigation across floating windows in nvim, here's a related issue. In order to make it happen it was decided to do some custom key-listening and intercepting user's windows movements.
• here's a short cursor glitch when switching between main and sidecar window due to the fact nvim has to return to "hidden" parrent window before getting back to floatings, I didn't come up with any sensible workaround for this, so fix is welcome.
  Other than that it should work smoothly, at least I've been already using it for a while and didn't find any experience-breaking issues.

Screenshots

[github-actions]

This PR is stale because it has been open 30 days with no activity.