gpui: Add a standard text example (#30747)

This is a dumb first pass at a standard text example. We'll use this to
start digging in to some text/scale rendering issues.

There will be a ton of follow-up features to this, but starting simple.

Release Notes:

- N/A

crates/gpui/src/gpui.rs

@@ -73,6 +73,8 @@ mod asset_cache;
 mod assets;
 mod bounds_tree;
 mod color;
+/// The default colors used by GPUI.
+pub mod colors;
 mod element;
 mod elements;
 mod executor;