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/script/dom/eventtarget.rs

@@ -377,7 +377,7 @@ impl CompiledEventListener {
                         ) {
                             let value = rooted_return_value.handle();
 
-                            //Step 5
+                            // Step 5
                             let should_cancel = value.is_boolean() && !value.to_boolean();
 
                             if should_cancel {