Add wasmJs support

[JavierSegoviaCordoba]

The code inside the Wasm source sets is a copy of the JS source sets. I don't know if it would be better to create an intermediate target (something like the web) or keep the duplication for the future.

[oldergod]

Looks good to me. I need Jake's eyes on it as well.

[oldergod]

Suggested change

	./gradlew -Dkjs=true -Dkwasm=true -Dknative=true -Pswift=false samples:native:build samples:js:build samples:multi-target:build --stacktrace --warning-mode all
	./gradlew -Dkjs=true -Dkwasm=true -Dknative=true -Pswift=false samples:native:build samples:js:build samples:wasm:build samples:multi-target:build --stacktrace --warning-mode all

[oldergod]

Suggested change

	./gradlew -Dkjs=false -Dkwasm=false -Dknative=false -Pswift=false build --stacktrace --warning-mode all -x samples:native:build -x samples:js:build -x samples:multi-target:build
	./gradlew -Dkjs=false -Dkwasm=false -Dknative=false -Pswift=false build --stacktrace --warning-mode all -x samples:native:build -x samples:js:build -x samples:wasm:build -x samples:multi-target:build

[JakeWharton]

I definitely don't think we should be duplicating the majority of files, especially when we'll also want to be adding wasmWasi as a supported target. The use of the hierarchy DSL should make it relatively trivial to add intermediate source sets based around functionality. It seems like we should have axes like "grpc" and "noGrpc" as well as "jvm" and "nonJvm" since that seems to be the source of most of the behavior differences which require the expect/actuals in the first place.