Allow specifying a custom limit for /search results (#12423)

<img width="497" alt="image"
src="https://github.com/zed-industries/zed/assets/482957/94e94326-fb3c-4f9b-b4d9-7dd6f6f8d537">


e.g.

```
/search --9 foobar
```

Release Notes:

- N/A

crates/theme/src/styles/syntax.rs

@@ -44,6 +44,11 @@ impl SyntaxTheme {
         self.get(name).color.unwrap_or_default()
     }
 
+    pub fn highlight_id(&self, name: &str) -> Option<u32> {
+        let ix = self.highlights.iter().position(|entry| entry.0 == name)?;
+        Some(ix as u32)
+    }
+
     /// Returns a new [`Arc<SyntaxTheme>`] with the given syntax styles merged in.
     pub fn merge(base: Arc<Self>, user_syntax_styles: Vec<(String, HighlightStyle)>) -> Arc<Self> {
         if user_syntax_styles.is_empty() {