ci: Cache npm dependencies

[relrelb]

This should improve CI build times.

Done as described in:
https://github.com/actions/setup-node#caching-packages-dependencies

[adrian17]

Do you think we could also cache built cargo dependencies - or at least wasm-bindgen binary? Because apparently cargo install wasm-bindgen-cli --version 0.2.78 alone takes 6-9m on windows machines.

[Herschel]

Caching wasm-bindgen binary should be a big help and straightforward.
Caching Rust dependencies is trickier because it's difficult to to differentiate things in the target folder, see: rust-lang/cargo#5885 rust-lang/cargo#7150
Worth investigating https://github.com/marketplace/actions/rust-cache , but this looks like it also just uploads target so may run afoul of the above issue.

[Herschel]

Thank you!