Emscripten compilation and execution in browsers

[j8asic]

This is a draft PR where SGL compiles using Emscripten.

It relies on RHI PR to be merged: shader-slang/slang-rhi#609 and Slang's PR shader-slang/slang#9818 to produce WASM libraries to be automatically used for RHI/SGL compilation

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[ccummingsNV]

This is exciting stuff and looks mostly sane to me, but I'd need @skallweitNV to take a look as he understands the build process better than me.

I don't think we can really ship this without it being part of the testing suite either. Currently almost all tests run on all platforms. How do we want to approach that.

[skallweitNV]

@j8asic thanks for the contribution. This is really cool to see! Let's first merge the slang-rhi PR and then get to this one after that.

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[j8asic]

thanks @ccummingsNV @skallweitNV; bumping to the latest slang version and merging the slang-rhi PR, this is almost ready for review.
one final question from my side: what does thread.h do, or better to say, what do you want it to do in the context of browsers?