Support tree shaking wasm in the rollup plugin

[brendandahl]

Describe the feature

When there are unused exports from a Wasm module they should be removed.

One possible way to do this:

1. Get the unused exports during generateBundle from module.removedExports
2. Read all the exports from the wasm file
3. Remove the unused exports from that list
4. Put those exports in a JSON file in the format that wasm-metacde (from binaryen) understands
5. Run wasm-metadce on the binary

FWIW, I've done something similar in a emscripten specific plugin here.

Additional information

• Would you be willing to help implement this feature?