feat!: init WASM during decode, remove top-level init

[pastelmind]

The initial implementation of WebAssembly-based decoder in #20 caused some ugly API changes. It required users to await a top-level init promise:

import PSD, {init} from '@webtoon/psd'

PSD.parse(/* ... */) // TypeError: undefined is not an object
init.then(() => {
  PSD.parse(/* ... */) // OK
})

Following a suggestion from @dlehdanakf, this PR removes the need for an init promise:

import PSD From '@webtoon/psd'

PSD.parse(/* ... */) // This works again, yay

Instead, the composite() method (available on Psd, Layer, and Group objects) is now async:

const psd = PSD.parse(/* ... */)
psd.composite().then((arr) => {
  /* ... */
})

Details

This PR extracts the WebAssembly-based image decoder into a separate package named @webtoon/psd-decoder. This package exports the init promise in addition to methods generated by wasm-pack.

The generateRgba() function in @webtoon/psd imports @webtoon/psd-decoder and awaits the init promise, ensuring that the WebAssembly module is ready before using any of its methods. This allows the rest of @webtoon/psd to work without worrying about init.

This PR effectively "moves" the top-level init from @webtoon/psd to @webtoon/psd-decoder. The asynchrony of WebAssembly is safely contained in generateRgba(), instead of contaminating the entire library.
On the flip side, generateRgba() is now asynchronous, even though it technically doesn't have to be...not yet, anyway.