text rendering: support strikethroughs (#7363)

<img width="1269" alt="image"
src="https://github.com/zed-industries/zed/assets/18583882/d4c93033-b2ac-4ae0-8e12-457f256ee869">

Release Notes:

- Added support for styling text with strikethrough.

Related: 
- https://github.com/zed-industries/zed/issues/5364
- https://github.com/zed-industries/zed/pull/7345

crates/outline/src/outline.rs

@@ -282,6 +282,7 @@ impl PickerDelegate for OutlineViewDelegate {
             line_height: relative(1.).into(),
             background_color: None,
             underline: None,
+            strikethrough: None,
             white_space: WhiteSpace::Normal,
         };