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/document.rs

@@ -1868,7 +1868,7 @@ impl Document {
         // TODO: Step 1, increase the event loop's termination nesting level by 1.
         // Step 2
         self.incr_ignore_opens_during_unload_counter();
-        //Step 3-5.
+        // Step 3-5.
         let beforeunload_event = BeforeUnloadEvent::new(
             &self.window,
             atom!("beforeunload"),
@@ -3267,7 +3267,7 @@ impl Document {
                     LazyLock::new(StyleSharedRwLock::new);
 
                 PER_PROCESS_AUTHOR_SHARED_LOCK.clone()
-                //StyleSharedRwLock::new()
+                // StyleSharedRwLock::new()
             },
             stylesheets: DomRefCell::new(DocumentStylesheetSet::new()),
             stylesheet_list: MutNullableDom::new(None),