Rename "wasm:js-boolean" "toI32"

[eqrion]

This is purely stylistic, but I think we could consider i32 to be the canonical wasm representation of a JS boolean. In which case we could call this operation "cast". I don't think we'd ever consider toI64 or toF32 for boolean operations, for example.

[sjrd]

The rationale for not choosing cast was that "js-string" "cast" still returns an externref. It only asserts that the argument is a string, but performs no conversion. If there was a "js-boolean" "cast", for consistency I would expect it to return an externref. (but that would not be useful in the context of this proposal).

We could still call this one cast by a "canonical Wasm representation" argument, but IMO that's not automatically a good choice.