[wasm] Modify workload to pick threading runtime packs (#72412)

[wasm] Modify workload to pick threading runtime packs (#72412)

* [wasm] Modify workload to pick threading runtime packs

This change adds the 2 wasm threading runtime packs to the wasm workload. In order for a threading runtime pack to be chosen, WorkloadManifest.targets is also modified to override the runtime pack name when the following props are set:

WasmEnableThreads - full threading support and will load Microsoft.NETCore.App.Runtime.multithread.Mono.browser-wasm

WasmEnablePerfTrace - runtime only threading support and will load Microsoft.NETCore.App.Runtime.perftrace.Mono.browser-wasm

* Add error condition when both WasmEnableThreading and WasmEnablePerfTrace are true

* [wasm] Set default value for MonoWasmBuildVariant, for runtime pack nuget

* [wasm] Wasm.Build.Tests: build runtime pack nugets with different names

- The earlier approach of simply making copies of the existing runtime
pack nuget with different names doesn't work, and `dotnet workload
install` rejects it.

```
  Installing pack Microsoft.NETCore.App.Runtime.Mono.multithread.browser-wasm version 7.0.0-ci...
  Workload installation failed. Rolling back installed packs...
```

Instead, now we build the missing nugets from the project with different
values for `$(MonoWasmBuildVariant)`.

- this handles local builds, and incremental builds also
- To skip building the missing nugets, for example, when you have all of
them available, then set `WasmSkipMissingRuntimePackBuild=true`.

* fix comment

Co-authored-by: Steve Pfister <steve.pfister@microsoft.com>
Co-authored-by: Ankit Jain <radical@gmail.com>