feat: defer and source phase instance imports support

[guybedford]

As the final semantic gap for the ESM integration instance semantics, here's the initial spec for how Wasm instances may be able to import dependencies with phases in the source or deferstage (both at implementation stage at TC39, with defer moving stabilization currently).

The gist of it is that we use the import name value which is not necessary in these import use cases to instead indicate the phase (import "foo" "wasm-js:phase/source" (global (ref extern))) or (import "foo" "wasm-js:phase/defer" (global (ref extern))).

We can then obtain:

• For defer, the namespace object directly, which can then be executed and used via a Reflect.get call.
• For source, The WebAssembly.Module instance as an extern for the source phase for Wasm (and in due course for JS).

Again we need to figure out if we want to use wasm-js here or just use wasm.

[nicolo-ribaudo]

This looks correct from the point of view of integrating with ECMA-262. I also personally like the proposed "syntax".

Is the idea that together with #116 I'd be able to do wasm-js:phase/source?foo=bar?

[guybedford]

Is the idea that together with #116 I'd be able to do wasm-js:phase/source?foo=bar?

Exactly, with this design both proposals could be used in conjunction with eachother in this way, without affecting the semantics of the other and aligning with JS semantics as well.