Fix some Closure warnings in Emscripten builds

[dcodeIO]

Instructs Closure about the types in play so the warnings go away. Didn't encounter the externs warning regarding the WebAssembly object (need to see what CI states). Generally it looks that we could refactor the code slightly to use classes, then we'd not need to annotate in comments so much (except the dynamically generated expression methods).

Related #5062

Update: Seen the WebAssembly object externs warning now, which might require a fix to the externs provided by Emscripten.

[dcodeIO]

Seems the temporary require fix does what is necessary. With it, the PR on the Emscripten end would not need to be reverted for Binaryen.js, even though it is likely that similar projects building to ESM would encounter the same problem. Could either:

1. Revert on the Emscripten side, which doesn't really fix the underlying issue but delays the problem long enough that it doesn't surface in Binaryen.js. Fix can be reverted on this PR.
2. Fix this properly on the Emscripten side as per this comment. Wait with this PR.
3. A combination of the two. Workaround for now as suggested here (which is safe, require will just become unused), and afterwards look into what can be done on the Emscripten end, so we can delete the externs-pre file here eventually.

Fine with what you folks decide, just need to know whether to revert the workaround here :)

[kripken]

Thanks!

[kripken]

This seems good for binaryen. Yeah, as you said, the general issue remains in emscripten, which we need to figure out - I worry other projects will hit this too. But for binaryen I think we're good now.