Port the picker and uniform list (#3248)

This adds a `UniformList` element and partially implements `Picker` as a
component, using `UniformList`. Because editor2 isn't fully implemented
yet, the picker doesn't have filtering logic yet. We want to merge this
for now though, to make the UniformList element available for other
crates.

Release Notes:

- N/A

crates/zed/src/languages/racket/highlights.scm

@@ -37,4 +37,3 @@
 
 ((symbol) @comment
  (#match? @comment "^#[cC][iIsS]$"))
-