Improve lsp semanticTokens range highlighting

Currently we debounce whenever the user scrolls the view. This results
in flickering sometimes, especially if the server is not very fast the
flickering can be annoying.

With this change, we optimize in a different way:
- Request highlighting for a bigger range than the current view range
- If the user scrolls, ignore it if the current highlighted range
  contains the scrolled region otherwise request new highlights
  immediately

This results in a much better user experience in my opinion.