Problem:
Nvim supports `textDocument/semanticTokens/full` and `…/full/delta`
already, but most servers don't support `…/full/delta` and Nvim will try
to request and process full semantic tokens response on every buffer
change. Even though the request is debounced, there is noticeable lag if
the token response is large (in a big file).
Solution:
Support `textDocument/semanticTokens/range`, which requests semantic
tokens for visible screen only.
23ddb2028b