Add CI for WASM

[Razican]

We have no checks whatsoever to make sure that the webassembly target is building properly. We should use wasm-pack to at least build it, and maybe to add some tests if it's possible.

[tomasol]

Hi, AFAK wasm-pack creates WASM files that can be executed only in a browser or on nodejs. However there are other areas (WASI, interface types) and runtimes like wasmtime which are useful for things like plugins.
I've actually succeeded to run boa using wasmtime, just by creating .Cargo/config with following:

[build]
target="wasm32-wasi"

BTW it produces a warning [1]. The wasm file can be executed:

wasmtime target/wasm32-wasi/debug/boa.wasm

and works as expected. It would be great if this was tested on CI as well.
[1]:

warning: output filename collision.
The bin target `boa` in package `boa_cli v0.8.0 (boa_cli)` has the same output filename as the lib target `boa` in package `Boa v0.8.0 (boa)`.
Colliding filename is: target/wasm32-wasi/debug/deps/boa.wasm
The targets should have unique names.
Consider changing their names to be unique or compiling them separately.
This may become a hard error in the future; see <https://github.com/rust-lang/cargo/issues/6313>.