Add a CI check for todo! and FIXME comments (#21950)

Motivation for this is to support writing comments that will certainly
be revisited before merge.

Release Notes:

- N/A

crates/repl/src/session.rs

@@ -610,7 +610,7 @@ impl Session {
 
                     // Start a new kernel
                     this.update(&mut cx, |session, cx| {
-                        // todo!(): Differentiate between restart and restart+clear-outputs
+                        // TODO: Differentiate between restart and restart+clear-outputs
                         session.clear_outputs(cx);
                         session.start_kernel(cx);
                     })