🔨 Make the Design System color tokens reachable from the rust code

The design system tokens are specified in the _base.scss file. To avoid to duplicate their value in a rust file, a new
step has been added to the building process to generate the `style_vars.rs` file which make the color tokens reachable
to the app.

Cargo.toml

@@ -31,3 +31,5 @@ futures = "0.3.29"
 [build]
 target = "x86_64-unknown-linux-gnu"
 
+[build-dependencies]
+regex = "1.10.3"