[wasm coreclr] plan app build infrastructure

[radekdoulik]

Look at current mono wasm build infra and also runtimelab/nativeaot to see how much we can reuse

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[maraf]

It should leverage the Wasm SDK (and underlaying StaticWebAssets SDK) as those solve various issues and situations related to web app development. NativeAOT-LLVM doesn't integrate with them and has some very simple one instead for supporting dotnet.js mode, the other mode is just plain emscripten setup