[dylink] Refactor loadWebAssemblyModule to avoid retaining wasm bytes. NFC (#28021)

Split `instantiateModule`, `addEmAsm`, and `addEmJs` out of
`loadWebAssemblyModule` and replace the async IIFE with `.then()` so
that long-lived import stub closures do not capture and retain the side
module's `binary` buffer in their shared closure context.

This also ends up saving a few bytes of code size which is nice.

See: #28002

Author: sbc100