Add #![forbid(unsafe_code)] where unsafe isn't needed (#2773)

To be consistent with the use of the safety badge in the README.

crates/egui_demo_lib/src/lib.rs

@@ -10,6 +10,7 @@
 
 #![allow(clippy::float_cmp)]
 #![allow(clippy::manual_range_contains)]
+#![forbid(unsafe_code)]
 
 mod color_test;
 mod demo;