Tooling · Workbench: a playground, and the gallery as the workbench

tooling::Playground lays a preview beside its knobs, each a switch, a
choice or a number that edits one property; the preview redraws from
the settings, which fail on a name or kind the knobs lack. The gallery
app is the ComponentGallery; its page 43 shows both.

src/lib.rs

@@ -41,6 +41,7 @@ pub mod shell;
 pub mod tables;
 pub mod terminal;
 pub mod theme;
+pub mod tooling;
 pub mod typography;
 
 pub use assets::Assets;