Support WASM with wasm_bindgen and conditional compilation

[spacebear21]

Migrated from LtbLightning/payjoin-ffi#46. See also related conversation in LtbLightning/payjoin-ffi#51

[w3irdrobot]

just FYI. i started messing with this last week. if someone else is working on it, that's cool. i just wanted to get an understanding of the size of the work.

my working branch: https://github.com/w3irdrobot/rust-payjoin/tree/wasm-capable

[spacebear21]

Closing this per #1190