Reproducible wasm builds

[zecuria]

We are looking to integrate the pdfium solution but with the wasm in particular, there are no build steps or reproduction instructions. With the other ones it appears to be integrating from https://github.com/bblanchon/pdfium-binaries but with the wasm the binary is just committed inline.

[jerbob92]

Hi! The WASM build is inline for deployment / integration speed, it is possible to load in your own WASM build in the WASM runtime using the Config.WASM property.

The issue with the WASM build is is that it's non-standard and requires a lot of patching to make it work. If you want to make it work you can use the following version of pdfium-binaries: bblanchon/pdfium-binaries#103

This pull request patches Emscripten and Pdfium to make it work in WASM standalone mode.
You can run ./build.sh -b chromium/7323 emscripten wasm-standalone to get a build for a specific version. Please be aware that if the version of your build doesn't match the version of the build in your go-pdfium version, you might run into errors on API methods marked as experimental.