Kakoune-like selection history

[kirawi]

Resolves #1596 by implementing a separate selection history. Each selection revision tracks the associated edit revision and selections are automatically mapped to different document revisions.

Waiting to see what should be done about transaction selections to finish this PR.

[kirawi]

I tried to take a look at how Kakoune implemented it, and while I didn't understand the edition terminology, it looked pretty similar to the regular history for edits so I did that and map selections to the current buffer. There are a few open questions I stumbled upon:

• Document::selections could be turned into HashMap<ViewId, SelectionHistory>
  • Was thinking of using an index to each selection, but wouldn't work for mapped selections.
• How should I handle the selections in each transaction? Should the field be removed?
• Is the History::between() function correct?

[kirawi]

Closing as stale