Blazor WebAssembly perf: RenderTreeArrayBuilder optimization

[SteveSandersonMS]

This is part of #22432

By introducing a new RenderTreeArrayBuilder type that combines the functionality of RenderTreeBuilder and ArrayBuilder, and by making RenderTreeFrame mutable, we can gain about 10% on the FastGrid benchmark. The improvement is due to reducing the amount of indirection and struct copying.

Prototype implementation is at fbd61ec

[SteveSandersonMS]

Done in #24484