Add a cargo machete CI step (#5171)

`cargo machete` looks for unused dependencies

tests/test_egui_extras_compilation/Cargo.toml

@@ -9,6 +9,8 @@ publish = false
 [lints]
 workspace = true
 
+[package.metadata.cargo-machete]
+ignored = ["eframe", "egui_extras"] # We don't use them, just check that things compile
 
 [dependencies]
 eframe = { workspace = true, features = ["default", "persistence"] }