Add WithRemSize element (#11928)

This PR adds a new `WithRemSize` element to the `ui` crate.

This element can be used to create an element tree that has a different
rem size than the base window.

`WithRemSize` can be nested, allowing for subtrees that have a different
rem size than their parent and their children.

<img width="912" alt="Screenshot 2024-05-16 at 2 25 28 PM"
src="https://github.com/zed-industries/zed/assets/1486634/f599cd9f-c101-496b-93e8-06e570fbf74f">

Release Notes:

- N/A

crates/storybook/src/story_selector.rs

@@ -40,6 +40,7 @@ pub enum ComponentStory {
     ToggleButton,
     ToolStrip,
     ViewportUnits,
+    WithRemSize,
 }
 
 impl ComponentStory {
@@ -76,6 +77,7 @@ impl ComponentStory {
             Self::ToggleButton => cx.new_view(|_| ui::ToggleButtonStory).into(),
             Self::ToolStrip => cx.new_view(|_| ui::ToolStripStory).into(),
             Self::ViewportUnits => cx.new_view(|_| crate::stories::ViewportUnitsStory).into(),
+            Self::WithRemSize => cx.new_view(|_| crate::stories::WithRemSizeStory).into(),
             Self::Picker => PickerStory::new(cx).into(),
         }
     }