Make BufferSearch less wide (#10459)

This also adds some "responsiveness" so that UI elements are hidden
before everything has to be occluded

Release Notes:

- Improved search UI. It now works in narrower panes, and avoids
scrolling the editor on open.

<img width="899" alt="Screenshot 2024-04-11 at 21 33 17"
src="https://github.com/zed-industries/zed/assets/94272/44b95d4f-08d6-4c40-a175-0e594402ca01">
<img width="508" alt="Screenshot 2024-04-11 at 21 33 45"
src="https://github.com/zed-industries/zed/assets/94272/baf4638d-427b-43e6-ad67-13d43f0f18a2">
<img width="361" alt="Screenshot 2024-04-11 at 21 34 00"
src="https://github.com/zed-industries/zed/assets/94272/ff60b561-2f77-49c0-9df7-e26227fe9225">
<img width="348" alt="Screenshot 2024-04-11 at 21 37 03"
src="https://github.com/zed-industries/zed/assets/94272/a2a700a2-ce99-41bd-bf47-9b14d7082b0e">

crates/ui/src/components/icon.rs

@@ -98,6 +98,7 @@ pub enum IconName {
     Plus,
     Public,
     Quote,
+    Regex,
     Replace,
     ReplaceAll,
     ReplaceNext,
@@ -196,6 +197,7 @@ impl IconName {
             IconName::Plus => "icons/plus.svg",
             IconName::Public => "icons/public.svg",
             IconName::Quote => "icons/quote.svg",
+            IconName::Regex => "icons/regex.svg",
             IconName::Replace => "icons/replace.svg",
             IconName::ReplaceAll => "icons/replace_all.svg",
             IconName::ReplaceNext => "icons/replace_next.svg",