* A bit of Generic DOM tidying/fixup
- Refactor out a test fixture for all DOM tests / benchmarks
- Optimize `GetType` implementation to not use `AZStd::variant::visit` (benchmark included to A/B the implementations)
- Tag a few more mutating Value functions with "Mutable" to avoid astonishing copy-on-writes
Benchmark results for GetType implementation:
```
DomValueBenchmark/AzDomValueGetType_UsingVariantIndex 18.2 ns 18.0 ns 40727273 items_per_second=443.667M/s
DomValueBenchmark/AzDomValueGetType_UsingVariantVisit 32.2 ns 32.2 ns 21333333 items_per_second=248.242M/s
```
Signed-off-by: Nicholas Van Sickle <nvsickle@amazon.com>
Nicholas Van Sickle
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