Introduce Editor::insert_flaps and Editor::remove_flaps (#12096)

This pull request introduces the ability to add flaps, custom foldable
regions whose first foldable line can be associated with:

- A toggle in the gutter
- A trailer showed at the end of the line, before the inline blame
information


https://github.com/zed-industries/zed/assets/482957/c53a9148-f31a-4743-af64-18afa73c404c

To achieve this, we changed `FoldMap::fold` to accept a piece of text to
display when the range is folded. We use this capability in flaps to
avoid displaying the ellipsis character.

We want to use this new API in the assistant to fold context while still
giving visual cues as to what that context is.

Release Notes:

- N/A

---------

Co-authored-by: Nathan Sobo <nathan@zed.dev>
Co-authored-by: Mikayla <mikayla@zed.dev>
Co-authored-by: Max <max@zed.dev>

crates/gpui/src/view.rs

@@ -280,6 +280,14 @@ impl<V: Render> From<View<V>> for AnyView {
     }
 }
 
+impl PartialEq for AnyView {
+    fn eq(&self, other: &Self) -> bool {
+        self.model == other.model
+    }
+}
+
+impl Eq for AnyView {}
+
 impl Element for AnyView {
     type RequestLayoutState = Option<AnyElement>;
     type PrepaintState = Option<AnyElement>;

crates/gpui/src/window.rs

@@ -4591,6 +4591,12 @@ impl From<(&'static str, u64)> for ElementId {
     }
 }
 
+impl From<(&'static str, u32)> for ElementId {
+    fn from((name, id): (&'static str, u32)) -> Self {
+        ElementId::NamedInteger(name.into(), id as usize)
+    }
+}
+
 /// A rectangle to be rendered in the window at the given position and size.
 /// Passed as an argument [`WindowContext::paint_quad`].
 #[derive(Clone)]