Add APIs for introspection of imports

As of right now, the current outward facing API (C, JS/TS, etc) allows you to enumerate all exports in a WASM module. These can be further introspected to get more specific info about that export.

However, there is no affordance for this on imports. This means the only real option is to enumerate all functions and see if each function is an import, enumerate all tables, etc. This is less than ideal.

In addition, for some reason there doesn't appear to be any way to determine whether memory is imported or not through the exposed APIs. There is also some additional metadata that would be useful on a few different object types that you can't get through the exposed APIs, which is tracked here: https://github.com/WebAssembly/binaryen/issues/3573. There a PR that was opened for that a while back but it looks like work on it stalled out before being merged.

Start by inspecting the outward-facing C and JS/TS APIs that currently enumerate exports, then compare how imported functions, tables, memories, and other objects are represented. The work is done when callers can introspect imports directly, determine whether memory is imported, and the additional metadata tracked in issue 3573 is accounted for.