Auto merge of #23507 - gterzian:introduce_browsing_context_groups, r=asajeffrey

Implement browsing context group and set

<!-- Please describe your changes on the following line: -->

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `___` with appropriate data: -->
- [ ] `./mach build -d` does not report any errors
- [ ] `./mach test-tidy` does not report any errors
- [ ] These changes fix #23307 (GitHub issue number if applicable)

<!-- Either: -->
- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because ___

<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/23507)
<!-- Reviewable:end -->

components/msg/constellation_msg.rs

@@ -166,6 +166,9 @@ impl fmt::Display for BrowsingContextId {
     }
 }
 
+#[derive(Clone, Default, Eq, Hash, PartialEq)]
+pub struct BrowsingContextGroupId(pub u32);
+
 thread_local!(pub static TOP_LEVEL_BROWSING_CONTEXT_ID: Cell<Option<TopLevelBrowsingContextId>> = Cell::new(None));
 
 #[derive(