Enable project search on type by default (#62506)

Closes https://github.com/zed-industries/zed/issues/9318

Most of the diff is tests.

Notable behavior changes during this mode, compared to current mode:

* unsaved changes in multi buffer won't spawn any "save or not" pop-ups
during typing — submitting the query still does it, as before.
Unsaved files will stay so after being sorted out on typing: we cannot
save them by default as file-less buffers need a save modal; Zed should
already be able to persist and restore such changes on restart.

* gutter may jump during search, if new excerpts with large line numbers
appear during query input — yet it would not shrink back to avoid
excessive flickering and rewraps

Release Notes:

- Enabled project search on type by default (use `{ "search": {
"search_on_type": false } }` to restore the previous behavior)

docs/src/reference/all-settings.md

@@ -3894,7 +3894,8 @@ Non-negative `integer` values
     "case_sensitive": false,
     "include_ignored": false,
     "regex": false,
-    "center_on_match": false
+    "center_on_match": false,
+    "search_on_type": true
   }
 }
 ```
@@ -3937,6 +3938,12 @@ Non-negative `integer` values
 - Setting: `center_on_match`
 - Default: `false`
 
+### Search On Type
+
+- Description: Start searching as you type in project search, without pressing Enter.
+- Setting: `search_on_type`
+- Default: `true`
+
 ## Search Wrap
 
 - Description: If `search_wrap` is disabled, search results do not wrap around the end of the file