editor: Improve selection highlights speed (#29097)

Before, we used to debounce selection highlight because it needed to
search the whole file to show gutter line highlights, etc. This
experience felt extremely laggy.

This PR introduces a new approach where:
1. We query only visible rows without debounce. The search function
itself is async and runs in a background thread, so it's not blocking
anything. With no debounce and such a small search space, highlights
feel realtime.
2. In parallel, we also query the whole file (still debounced, like
before). Once this query resolves, it updates highlights across the
file, making scrollbar markers visible.

This hybrid way gives the feeling of realtime, while keeping the same
functionality.


https://github.com/user-attachments/assets/432b65f1-89d2-4658-ad5e-048921b06a23

P.S. I have removed the user setting for custom debounce delay, because
(one) now it doesn't really make sense to configure that, and (two) the
whole logic is based on the assumption that the fast query will resolve
before the debounced query. A static debounce time makes sure of that.
Configuring it might lead to cases where the fast query resolves after
the debounced query, and we end up only seeing visible viewport
highlights.

Release Notes:

- Improved selection highlight speed.

docs/src/configuring-zed.md

@@ -536,13 +536,6 @@ List of `string` values
 - Setting: `selection_highlight`
 - Default: `true`
 
-## Selection Highlight Debounce
-
-- Description: The debounce delay before querying highlights based on the selected text.
-
-- Setting: `selection_highlight_debounce`
-- Default: `50`
-
 ## LSP Highlight Debounce
 
 - Description: The debounce delay before querying highlights from the language server based on the current cursor location.