Add web backend

[kvinwang]

There is an issue with the wasm-bindgen tool for generating JavaScript bindings:

wasm-bindgen phatjs-web.wasm  --out-dir . --typescript --target web --out-name index
[2023-12-11T13:14:16Z WARN  walrus::module] in name section: index `0` is out of bounds for data
[2023-12-11T13:14:16Z WARN  walrus::module] in name section: index `1` is out of bounds for data
thread 'main' panicked at 'unknown instruction Block(Block { seq: Id { idx: 1 } })', /home/kvin/.cargo/registry/src/index.crates.io-6f17d22bba15001f/wasm-bindgen-wasm-interpreter-0.2.89/src/lib.rs:367:18
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

More info: rustwasm/wasm-bindgen#3315