Fix WASM build and add a job in CI

[RCasatta]

close #194

[RCasatta]

Note downstream you can use this without the dev-dep obviously, this is just needed to enforce wasm check in CI so that we are not breaking it in the future

[apoelstra]

LGTM, but would this check have actually caught the getrandom thing? My understanding is that if you compile without the js feature you'll get a runtime panic, so just running cargo check won't have noticed it.

Regardless, this CI job is an improvement over having nothing.

[RCasatta]

It seems it's a compile time check and without the getrandom thing the job would fail https://github.com/RCasatta/rust-elements/actions/runs/10578325188/job/29308193852?pr=2