This commit will eliminate flicker while editing one open window. It
works by querying previously calculated trees for highlight marks, in
the case that we are still asynchronously parsing in `on_win`.
It will not fully solve the case of flicker when there are multiple open
windows, since the parser will drop previously parsed injection trees
whenever it receives a call to parse a different region of the buffer.
This will require a refactor of `languagetree.lua`.
(cherry picked from commit 8d75910ef9)
5c6ee251a6