Problem:
If users delete a line containing extmark, it will move to the next
line, which could highlight the next line in an unwanted way.
Solution:
- Use `invalidate` field in `nvim_buf_set_extmark()` to prevent the
extmark from moving.
- Also save from "priority" hacking, since we can check if the extmark
is valid in `nvim_buf_get_extmarks()` now.
64afa93187
