Add CI for WASM targets

[newpavlov]

In #180 we have migrated to GitHub Actions almost all tests except related to WASM and Emscripten.

Old Travis config

[josephlr]

With the stdweb support removed in #178, this should now be easier to get working. The best approach might be to have a Dockerfile containing the Node/Firefox/Chromium deps, so that we can keep all of those ordered in a single place.