Add search_on_input setting to Project Search (#42889)

I was really missing the ability to instantly see search results while
typing in the Project Search and decided to try and implement it.
As this may not be a feature that _everyone_ wants, I made it
toggle-able via the settings (or the settings.json)
### Settings
Set to false by default
<img width="911" height="618" alt="Screenshot 2025-11-17 at 16 17 09"
src="https://github.com/user-attachments/assets/8eaaab65-684e-4c5f-9a3c-9cb62cff0925"
/>
### settings.json
Set to false by default
<img width="396" height="193" alt="Screenshot 2025-11-17 at 16 18 21"
src="https://github.com/user-attachments/assets/90ebda95-c454-4bc5-8423-5da593832fd2"
/>

### Video demo:

https://github.com/user-attachments/assets/715d6b77-3a61-45f8-8e1a-9bd880c697c3

- Search input is debounced with 250ms in this mode (cool?)


The desire for this feature has been expressed here too:
https://github.com/zed-industries/zed/discussions/30843

Release Notes:

- Enabled project search on input. Can be turned off with
`search.search_on_input` settings toggle.

docs/src/reference/all-settings.md

@@ -3270,6 +3270,12 @@ Non-negative `integer` values
 - Setting: `regex`
 - Default: `false`
 
+### Search On Input
+
+- Description: Whether to search on input.
+- Setting: `search_on_input
+- Default: `true`
+
 ### Center On Match
 
 - Description: Whether to center the cursor on each search match when navigating.
@@ -3282,12 +3288,6 @@ Non-negative `integer` values
 - Setting: `search_wrap`
 - Default: `true`
 
-## Center on Match
-
-- Description: If `center_on_match` is enabled, the editor will center the cursor on the current match when searching.
-- Setting: `center_on_match`
-- Default: `false`
-
 ## Seed Search Query From Cursor
 
 - Description: When to populate a new search's query based on the text under the cursor.