Initial support for WASM build in CMake

[Kangz]

[illwieckz]

I'm marking this as draft to make it easy for me to list PR that have higher review priority, but I would like to see it merged so we can move on and use this as a basis for other things. @Kangz can you look at @slipher's comment? Feel free to remove the draft state at any time, it is only there to help me listing stuff.