Button2 – Part1 (#3420)

## TODO

- [x] Remove `InteractionState`
- [ ] `Selectable` should use `Selection` instead of a boolean
- [x] Clean out ui2 prelude
- [ ] Build out button2 button types
- [ ] Port old buttons

Release Notes:

- N/A

---------

Co-authored-by: Marshall Bowers <1486634+maxdeviant@users.noreply.github.com>

crates/ui2/src/fixed.rs

@@ -0,0 +1,6 @@
+use gpui::DefiniteLength;
+
+pub trait FixedWidth {
+    fn width(self, width: DefiniteLength) -> Self;
+    fn full_width(self) -> Self;
+}