Add CI to build on WebAssembly

[jeroen]

I am trying to build this package for emscripten. Currently it fails with:

no variant or associated item named `Path` found for enum `FrameSource` in the current scope

Perhaps there is a way to support emscripten VFS?

This PR adds the CI to reproduce the error.

[kornelski]

I think dependency on filesystem paths can be easily removed.

However, do you have support for threads?

This library is architected to run on multiple threads interacting with each other, and that's not easy to change.

[jeroen]

We enable threads in Emscripten by passing the -pthread flag as both compile and linker flag.

IIUC, we get threads support in rust by adding -Zbuild-std=panic_abort,std (which also may require installing the emscripten toolchain). For example we use these flags to compile polars:

https://github.com/pola-rs/r-polars/blob/02c90c5d02ad61d076207fdcc463172464541279/src/Makevars.in#L33-L38

[jeroen]

I have updated this PR to build with -Zbuild-std so I think it should now work once the filepaths issue is addressed.