feat!: implement Rust/WebAssembly-based decoder

Reimplement the image decoder, previously implemented in handwritten
Asm.js, into Rust/WebAssembly. Since Asm.js is effectively a "done"
technology and is unsupported by esbuild, moving to WebAssembly is a
sensible choice.

The new implementation is almost as fast as the previous one in Chrome.

At build time, the WebAssembly binary is inlined into the bundle as a
base64-encoded string. This allows downstream users to use regular
bundlers.

Due to how WebAssembly works, however, the library's signature has
changed.
Now, the library exports a `Promise` object named `init`. This promise
is created when the library is imported, and is resolved asynchronously.
Before using symbols exported by the library (including the default
export), users must wait for the promise to resolve.

example:

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

    async fun() => {
      await init;
      PSD.parse(/* ... */)
    }

.prettierignore

@@ -1,3 +1,5 @@
 /coverage/
 dist/
 /dist-web/
+**/rust-wasm/pkg/
+**/rust-wasm/target/