Option to embed .wasm as a base64 string in JavaScript file

[mseele]

Motivation

Embed the .wasm file as a base64 string in JavaScript (like described here and requested in wasm-pack two times: rustwasm/wasm-pack#831 & rustwasm/wasm-pack#1074).
This would be useful in scenarios where the JS engine is embedded as a scripting environment as the concept of "reading files from disk" does not work.
Another useful example is packaging the js code into a single file with esbuild, e.g. for aws lambda.

Proposed Solution

Let's add a new --inline option that will embed the .wasm file as a base64 string in JavaScript.

Alternatives

I don't have a good alternative right now. If anyone has a better idea, please comment.

[daxpedda]

I don't see us adding both features in wasm-bindgen, because it can be done externally.

Unfortunately the Web is a platform with extremely diverse needs, so I'm generally against implementing a solution to every possible requirement out there and would rather see wasm-bindgen enabling other tools to be built around it, e.g. wasm-pack.

If wasm-bindgen has issues supporting external solution to this problem I'm more then happy to review a PR fixing or implementing something towards that.