Fix documentation labels obscuring other content in completion menu (#4167)

This PF fixes an issue where the documentation labels in the completion
menu could end up completing obscuring the primary content.

#### Before
<img width="691" alt="Screenshot 2024-01-19 at 1 42 19 PM"
src="https://github.com/zed-industries/zed/assets/1486634/187f2978-08be-4f5e-ba93-91ed09cd6e87">

#### After

<img width="704" alt="Screenshot 2024-01-19 at 1 39 45 PM"
src="https://github.com/zed-industries/zed/assets/1486634/e5929c04-bace-4336-b878-4ada5660d235">

Since this involved changes to the `ListItem`, I also made sure to test
the other problematic case that was fixed in #3845 to make sure we
didn't regress there:

<img width="665" alt="Screenshot 2024-01-19 at 1 39 11 PM"
src="https://github.com/zed-industries/zed/assets/1486634/14b72d44-b6a6-4aef-a708-37fb60b8875a">

I also tried to capture these cases in the `ListItem` stories to make it
easier to test all of them at once:

<img width="1141" alt="Screenshot 2024-01-19 at 1 40 03 PM"
src="https://github.com/zed-industries/zed/assets/1486634/333886ff-e394-4cba-89db-98352481c7f6">

Release Notes:

- Fixed an issue where documentation labels could obscure other content
in the editor completion menu
([#2419](https://github.com/zed-industries/community/issues/2419)).

crates/gpui/src/styled.rs

@@ -307,6 +307,13 @@ pub trait Styled: Sized {
         self
     }
 
+    /// Sets the initial size of flex items for this element.
+    /// [Docs](https://tailwindcss.com/docs/flex-basis)
+    fn flex_basis(mut self, basis: impl Into<Length>) -> Self {
+        self.style().flex_basis = Some(basis.into());
+        self
+    }
+
     /// Sets the element to allow a flex item to grow to fill any available space.
     /// [Docs](https://tailwindcss.com/docs/flex-grow)
     fn flex_grow(mut self) -> Self {
@@ -314,6 +321,20 @@ pub trait Styled: Sized {
         self
     }
 
+    /// Sets the element to allow a flex item to shrink if needed.
+    /// [Docs](https://tailwindcss.com/docs/flex-shrink)
+    fn flex_shrink(mut self) -> Self {
+        self.style().flex_shrink = Some(1.);
+        self
+    }
+
+    /// Sets the element to prevent a flex item from shrinking.
+    /// [Docs](https://tailwindcss.com/docs/flex-shrink#dont-shrink)
+    fn flex_shrink_0(mut self) -> Self {
+        self.style().flex_shrink = Some(0.);
+        self
+    }
+
     /// Sets the element to align flex items to the start of the container's cross axis.
     /// [Docs](https://tailwindcss.com/docs/align-items#start)
     fn items_start(mut self) -> Self {