tidy: Add a rule ensuring that // comments are followed by a space in Rust (#38698)

This shows up sometimes in code reviews, so it makes sense that tidy
enforces it. `rustfmt` supports this via comment normalization, but it
does many other things and is still an unstable feature (with bugs).

Testing: There are new tidy tests for this change.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/devtools/lib.rs

@@ -527,7 +527,7 @@ impl DevtoolsInstance {
         let mut actors = self.actors.lock().unwrap();
         match self.actor_requests.entry(request_id) {
             Occupied(name) => {
-                //TODO: Delete from map like Firefox does?
+                // TODO: Delete from map like Firefox does?
                 name.into_mut().clone()
             },
             Vacant(entry) => {