Introduce dynamic tab titles for unsaved files based on buffer content (#32353)

https://github.com/user-attachments/assets/0bb08784-251c-4221-890a-2d6b3fb94e0f

For new, unsaved files:

- If a buffer has no content, or contains only whitespace, use
`untitled`
- If a buffer has content, take the first 40 chars of the first line

| Sublime | VS Code | Zed |
|---------|---------|-----|
| <img width="227" alt="SCR-20250608-ouux"
src="https://github.com/user-attachments/assets/d02b1e50-5775-4252-86e6-6c9d3f6c72fb"
/> | <img width="230" alt="SCR-20250608-ousn"
src="https://github.com/user-attachments/assets/7c9c016b-642f-4a80-9bc1-8c9bdc7bbd32"
/> | <img width="242" alt="SCR-20250608-ovbg"
src="https://github.com/user-attachments/assets/c7f4be5c-5bba-4a2a-b477-1392ca938cd5"
/> |

Note that this implementation also trims all leading whitespace, so that
if the buffer has any non-whitespace content, we use it. VS Code and
Sublime do not do this.

| Sublime | VS Code | Zed |
|---------|---------|-----|
| <img width="233" alt="SCR-20250608-oviq"
src="https://github.com/user-attachments/assets/ccffecc6-0f46-4d1b-8739-740240bc067b"
/> | <img width="198" alt="SCR-20250608-ovkq"
src="https://github.com/user-attachments/assets/35c20149-f898-417b-aff3-dda22b8cc1f3"
/> | <img width="233" alt="SCR-20250608-ovns"
src="https://github.com/user-attachments/assets/2509e8f6-254b-4fcb-a0ea-e18e95bb685b"
/> |

Release Notes:

- Introduced dynamic tab titles for unsaved files based on buffer
content

crates/zed/src/zed.rs

@@ -3027,7 +3027,7 @@ mod tests {
         });
         cx.read(|cx| {
             assert!(editor.is_dirty(cx));
-            assert_eq!(editor.read(cx).title(cx), "untitled");
+            assert_eq!(editor.read(cx).title(cx), "hi");
         });
 
         // When the save completes, the buffer's title is updated and the language is assigned based