Add new "splice" interface for tracking buffer changes at the byte
level. This will later be reused for byte-resolution buffer updates.
(Implementation has been started, but using undocumented "_on_bytes"
option now as interface hasn't been finalized).
Use this interface to improve many edge cases of extmark adjustment.
Changed tests indicate previously incorrect behavior. Adding tests for
more edge cases will be follow-up work (overlaps on_bytes tests)
Don't consider creation/deletion of marks an undoable event by itself.
This behavior was never documented, and imposes complexity for little gain.
Add nvim__buf_add_decoration temporary API for direct access to the new
implementation. This should be refactored into a proper API for
decorations, probably involving a huge dict.
fixes#11598
NB: this is not the final implementation. Bufhl should be made a
part of the extmark tree, so that "start" adjustment just works
automatically. But "stop" will still need some ad-hoc trickery,
until extended marks natively support ranges (hopefully sooner than
forever).
