Enable shipping WebAssembly source code alongside main Snap bundle

Snaps that use WebAssembly (WASM) currently have two options for loading their WASM source at runtime:
1. Host it somewhere and fetch it at runtime (e.g. [here](https://github.com/0xPARC/cabal/blob/ebab0001cfdf7ebac8b44153e56e0c3dd3190deb/snap/src/snark_utils/generate_proof.js))
2. Turn the WASM source into a byte string, bundle that into the JavaScript source and convert it to a buffer at runtime

(1) requires network access - which makes it harder to trust the snap - while (2) is clunky and may make the JavaScript bundle impractically large. We need to provide a way for snaps to ship their `.wasm` files alongside the main JavaScript bundle and load them at runtime.

No files or tests are named. Start by tracing how the main JavaScript bundle is produced and how Snaps load runtime resources, then define the packaging and loading path for a separate .wasm file. Done means a Snap can ship its WASM source alongside the bundle and load it without network access or embedding it as a byte string.

Written by the indexing model from the issue text.