feat(wasm): support source phase imports and enable js-string builtins

[guybedford]

Follow-on to the WASM ESM Integration work, adding support for the source phase import proposal for .wasm files. A source phase import hands you the compiled WebAssembly.Module without instantiating it, so you own instantiation entirely — the standard alternative to ?init for instantiation control:

import source mod from './mod.wasm'

const instance = await WebAssembly.instantiate(mod, {
  './imports.js': { someFunc: () => {} },
})

The dynamic import.source('./mod.wasm') form resolves to the WebAssembly.Module as well.

Rather than rely on native import source support (which isn't available across all supported browsers or Node versions yet), this polyfills the proposal on top of Vite's existing module pipeline:

• import source x from './m.wasm' is rewritten to a plain import x from './m.wasm?source', whose module resolves to a compiled WebAssembly.Module.
• import.source('./m.wasm') becomes import('./m.wasm?source').then((m) => m.default), unwrapping the namespace default to match the native semantics where the promise resolves to the module source object.
• In SSR the ?source module compiles via node:fs, so it works on every supported Node version regardless of whether the runtime understands the syntax. We can drop the polyfill and pass import source through natively once Node 22 reaches EOL (April 2027).

This also enables the JS String Builtins and Imported String Constants proposals on compile/instantiate, matching the WebAssembly/ES Module Integration loader.

Tests cover static and dynamic source phase imports, and source phase imports instantiated with the wasm module's own imports, in both serve and build. Docs are updated with a new "Source Phase Imports" section.