Commit undo checkpoints before each :write

[the-mikedavis]

This fixes the modification indicator when saving from insert mode with a config such as:

[keys.insert]
C-s = ":write"

Previously the modification indicator would be stuck showing modified even if the buffer contents matched the disk contents when writing after some changes in insert mode with this binding. In insert mode we do not eagerly write undo checkpoints so that all changes made become one checkpoint as you exit insert mode. When saving, Documents changes ChangeSet would be non-empty and when there are changes we show the buffer as modified. Then switching to normal mode would append the changes to history, bumping the current revision past what it was when last saved. Since the last saved revision and current revision were then unsynced, the modification indicator would always show modified.

This matches Kakoune's behavior. Kakoune has a different architecture for writes but a very similar system for history, transactions and undo checkpoints (what it calls "undo groups"). Upon saving Kakoune creates an undo checkpoint if there are any uncommitted changes. It does this after the write has gone through since its writing system is different. For our writing system it's cleaner to make the undo checkpoint before performing the save so that the history revision increments before we send the save event.

Fixes #11024
Closes #7226

I have also reverted #11047 in this PR since delaying :writes until leaving insert mode is no longer necessary. I believe that not saving in insert mode weakens the auto-save-after-delay feature considerably.

[the-mikedavis]

Ideally Document::save_impl would be in charge of this but currently Document::append_changes_to_history needs a &mut View. Maybe we can refactor that later and move this call into document.

[the-mikedavis]

On second though I dropped the commit to revert #11047. @pascalkuthe and I discussed it and reverting it would mean making potentially many of undo checkpoints in insert mode. If we can find a clever way to track modification status without undo checkpoints we could restore the "save in insert mode" behavior for auto-save.