Add wiring for UI density (#11260)

Note: You shouldn't use the `unstable.ui_density` setting – it is only
being added for testing and to enable new UI components to be built with
density in mind. Don't expect this to work well, or at all right now.

Adds some of the basic wiring we'll need to start scaling UI elements
throughout the app based on a desired density setting.

Release Notes:

- N/A

crates/ui/src/styles.rs

@@ -1,11 +1,13 @@
 mod color;
 mod elevation;
 mod platform;
+mod spacing;
 mod typography;
 mod units;
 
 pub use color::*;
 pub use elevation::*;
 pub use platform::*;
+pub use spacing::*;
 pub use typography::*;
 pub use units::*;