* Add `no_trace` option to JSTraceable derive
* NoTrace wrapper
* Port some types to no_trace schematics
* Fixing my unsafe mistakes (not tracing traceables)
* Add docs & safety guards for no_trace
Safety guards (trait shenanigans) guarantees safety usage of `no_trace`
* Port canvas_traits to no_trace
* Port servo_media to no_trace
* Port net_traits to no_trace
* Port style to no_trace
* Port webgpu to no_trace
* Port script_traits to no_trace
* Port canvas_traits, devtools_traits, embedder_traits, profile_traits to no_trace
* unrooted_must_root lint in seperate file
* Add trace_in_no_trace_lint as script_plugin
* Composable types in must_not_have_traceable
* Introduced HashMapTracedValues wrapper
* `HashMap<NoTrace<K>,V>`->`HashMapTracedValues<K,V>`
* Port rest of servo's types to no_trace
* Port html5ever, euclid, mime and http to no_trace
* Port remaining externals to no_trace
* Port webxr and Arc<Mutex<_>>
* Fix spelling in notrace doc
Add a pref to indicate user intent to enter webxr
<!-- Please describe your changes on the following line: -->
Add a pref to indicate that the user has indicated intent to enter webxr.
---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `___` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes do not require tests because it's a command-line pref
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