Source Phase Imports support

[guybedford]

Describe the solution

We support source phase imports for WebAssembly in workerd since last year now.

This provides a standards-compatible way to import WebAssembly as:

import source mod from './mod.wasm';

whereas the default import form we use today is effectively non-standard:

import mod from './mod.wasm';

Migrating users over to the souce phase imports form will allow us a path to align on standards-compatible Wasm ESM Integration.

The only thing stopping this from working today in wrangler workflows is that the es2024 target does not include the Stage 3 Source Phase Imports spec. esbuild will support these when configured as externals though. Disabling bundling doesn't seem to help here unfortunately.

I'd like to suggest we amend our target to ensure first-class support for source phase imports of WebAssembly as we migrate to the ESM Integration standard (https://github.com/webassembly/esm-integration).