protoc-gen-grpc-web is not available from npm

There is an unofficial package on npm https://www.npmjs.com/package/protoc-gen-grpc-web, which bootstraps protoc-gen-grpc-web by downloading it from github's release page.

This is not not ideal, as it would require opening up build servers to the internet rather than simply mirroring a package manager feed.

I suggest that protoc-gen-grpc-web be cross compiled to asm.js/wasm, which can then be packaged into npm and be run from any node.js capable machine (given that you are running `npm install`, that is almost guaranteed to be the case).