Start work on creating gpui2 version of project panel (#3299)

I'm gonna land what I have, even though some features aren't ported yet,
since we're working on all of this code so actively.

* [x] get the basic structure compiling
* [x] get the panel laying out correctly
* [ ] rename / new file editor
* [ ] enable the tests
* [ ] drag and drop
* [ ] context menu

crates/gpui2/src/color.rs

@@ -293,7 +293,16 @@ pub fn blue() -> Hsla {
 
 pub fn green() -> Hsla {
     Hsla {
-        h: 0.3,
+        h: 0.33,
+        s: 1.,
+        l: 0.5,
+        a: 1.,
+    }
+}
+
+pub fn yellow() -> Hsla {
+    Hsla {
+        h: 0.16,
         s: 1.,
         l: 0.5,
         a: 1.,