build: Add js Cargo feature flag for WASM builds

[lpahlavi]

Previously, the wasm32 target implicitly assumed a browser environment, which caused issues when building for non-browser WASM environments due to the unconditional inclusion of wasm-bindgen.

This commit introduces an explicit 'js' feature flag, making wasm-bindgen and js-sys conditional dependencies. This allows greater flexibility for different WASM execution environments.

Related to #47

[lpahlavi]

@joncinque This is essentially the same as the PR in the solana-sdk repository.

[joncinque]

This one should be easier to merge since it only affects the interface crate in this repo. I think we should move wasm.rs into a new crate, maybe we call it solana-system-interface-wasm-js?

@LucasSte what do you think?

[lpahlavi]

@joncinque Thanks for looking into this! If it is confirmed that adding a new solana-system-interface-wasm-js crate is the way to go here, should I take the lead on this or would someone from Anza?

[joncinque]

If you have the time, please go for it!

[LucasSte]

This one should be easier to merge since it only affects the interface crate in this repo. I think we should move wasm.rs into a new crate, maybe we call it solana-system-interface-wasm-js?

@LucasSte what do you think?

Yes, that would be cool. I noticed that in this repo we also mix crate types (cdylib and rlib) for the same crate. I haven't checked why we need both yet, but I wanted them not be used together (it hinders LTO).

[joncinque]

I noticed that in this repo we also mix crate types (cdylib and rlib) for the same crate. I haven't checked why we need both yet, but I wanted them not be used together (it hinders LTO).

Agreed, we definitely need to change that too

[lpahlavi]

@joncinque @LucasSte I took a stab at doing something similar here as in the solana-sdk repository. Since I obviously cannot have an impl block for SystemInstruction in the new WASM crate but all the exported methods were static, I simply removed the impl block, which I believe should result in the same behaviour as before. I am not a wasm_bindgen expert though so PTAL and confirm that this is indeed correct.