[wasm] Source Phase Imports for ESM Integration

[guybedford]

This adds a test for the new source phase imports aspect of the ESM integration.

Includes tests both for using the source phase, and for checking that the prototype behaviours match the updated requirements on WebAssembly.Module extending AbstractModuleSource.

Per the note in https://github.com/WebAssembly/esm-integration#es-module-integration-proposal-for-webassembly, it is possible for implementations to implement source phase imports independently from non-source-phase imports. I don't know if there's a way we want to reflect that in the test structure though?

[lucacasonato]

This can be rewritten as a .any.js file which will make this also runnable from Web Workers and Service Workers.

[Ms2ger]

Can it? Note <script type=module>. The docs seem to show that the worker scopes have a module variant, but window doesn't.

[guybedford]

I've split out the tests to separate the module source constructor tests from the HTML ones, so they should work across different contexts. I've also extended the ESM integration HTML worker test to include source phase imports. PTAL