Problem:
diagnostic extmark used for positioning continues to exist after
deleting a range containing it, so it's possible to jump to a
next/previous diagnositc, which isn't visible in any way, including not
being shown via `open_float`.
Solution:
enable `invalidate` flag when setting an extmark to be able to filter
out diagnostics based on `invalid` flag when looking for next/previous
diagnostic to jump to.
7a051a4c38
