Tracking issue - WASM_BINDGEN snippets support changes

This is a tracking issue from https://github.com/emscripten-core/emscripten/pull/27208#discussion_r3929655457.

wasm-bindgen outputs directly to a snippets folder for external inline code snippets, but does not optimize these into the single JS file itself. As a result this folder must be copied with the wasm-bindgen output files.

These are distinct from Emscripten-style inline code because they assume module syntax like an ES module file themselves - supporting import 'dep' and using their export interface as the snippet interface.

In future two possible optimizations could arise here:

1. Emscripten doing the single-file JS optimization to inline these static known imports
2. wasm-bindgen adding support for this optimization itself

In either of these cases, copying the snippets directory to support inline code would no longer be need which this tracks.