ui2: Reference theme2 and settings2 crates without the 2 (#3424)

This PR updates the `ui2` crate to reference the `theme2` and
`settings2` crates without the `2` the way we do in our other crates.

Release Notes:

- N/A

crates/ui2/src/styled_ext.rs

@@ -1,6 +1,6 @@
 use gpui::{px, Styled, WindowContext};
-use theme2::ActiveTheme;
 
+use crate::prelude::*;
 use crate::{ElevationIndex, UITextSize};
 
 fn elevated<E: Styled>(this: E, cx: &mut WindowContext, index: ElevationIndex) -> E {