Check if represented attributes are cached
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Improve existing cache checks in `/bluetooth/lib.rs`, and update Step comments in `script/dom` corresponding to this.
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
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It could be used to have mutable JSVal fields without GC barriers.
With the removal of that trait, MutHeap and MutNullableHeap can respectively
be replaced by MutJS and MutNullableJS.
