tab_switcher: Add tab close buttons (#9968)

Support for closing tabs from Tab Switcher:

- Close button color matches the indicator color to preserve the
information that the buffer is dirty (as in SublimeText).
- `ctrl-backspace` closes the currently selected item.


https://github.com/zed-industries/zed/assets/2101250/8ea33911-2f62-4199-826d-c17556db8e9a

Release Notes:

- N/A

crates/ui/src/components/indicator.rs

@@ -13,7 +13,7 @@ pub enum IndicatorStyle {
 pub struct Indicator {
     position: Position,
     style: IndicatorStyle,
-    color: Color,
+    pub color: Color,
 }
 
 impl Indicator {