editor: Add debounce setting for triggering DocumentHighlight (#21702)

Closes https://github.com/zed-industries/zed/issues/6843


I don't see where is the logic to remove old document highlight when new
one applies,
ideally, old highlight should be cleared as soon as possible when cursor
moves if the new position does not
sits in old highlight ranges to avoid linger highlights described in
https://github.com/zed-industries/zed/issues/13682#issuecomment-2498368680.

So current solution is still not ideal, because only when lsp responses
highlight ranges (even is a empty set) can we clear the old one.

Release Notes:

- Added a setting `lsp_highlight_debounce` to configure delay for
querying highlights from language server.

---------

Co-authored-by: mgsloan@gmail.com <michael@zed.dev>

assets/settings/default.json

@@ -144,6 +144,9 @@
   // 4. Highlight the full line (default):
   //    "all"
   "current_line_highlight": "all",
+  // The debounce delay before querying highlights from the language
+  // server based on the current cursor location.
+  "lsp_highlight_debounce": 75,
   // Whether to pop the completions menu while typing in an editor without
   // explicitly requesting it.
   "show_completions_on_input": true,