Add bindings to support Wasm WASI.

[ajalt]

Hi, thanks for making this great library!

I'm trying to use this library on the wam32-wasip1 target to run on a standalone Wasm runtime like Wasmtime. I can compile against that target, but it looks like all the code in src/wasm.js depends on the JS shims generated by wasm_bindgen, which aren't available on WASI.

Assuming I already have a WASI-compiled pdfium library, do you have an idea of what it would take to link pdfium-render to it? Is that even possible, or would it require changes in pdfium-render?