Emit polyfill-nomodule.js into the build manifest polyfillFiles

[bgw]

Why?

polyfill-nomodule.js is a pre-built file containing polyfills for older browsers (gated by the <script> tag nomodule attribute).

How?

• The turbopack server needs to emit a raw OutputAsset for this file, so that it is copied into the output chunks directory.
• That file needs to be passed into polyfillFiles, and preserved when we're merging manifests inside of the development server.

Test Plan

HEADLESS=true pnpm testonly-dev test/e2e/app-dir/app/index.test.ts -t 'should serve polyfills for browsers that do not support modules'
HEADLESS=true pnpm testonly-dev-turbo test/e2e/app-dir/app/index.test.ts -t 'should serve polyfills for browsers that do not support modules'

Build a project with next dev --turbo and inspect:

Verify that the polyfill file exists and resolves in the browser.

Closes PACK-2993

[bgw]

• [turbopack] Use updated RawOutput API #65300
• Emit polyfill-nomodule.js into the build manifest polyfillFiles #65223 👈
• canary

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[sokra]

It doesn't really matter here, but you could change it to:

Suggested change

	let polyfill_output_asset =
	VirtualOutputAsset::new(polyfill_output_path, polyfill_module.content());
	let polyfill_output_asset =
	RawOutput::new(polyfill_output_path, polyfill_module);

If you give RawOutput an additional path argument, which it reports as ident() AssetIdent::from_path(path). That would allow the RawOutput to "serve" a Source under a specific path.

---

The motivation behind that:

Eventually we want to have lazy compilation. That would mean instead of emitting all OutputAssets directly to disk we would directly serve them on request. That would mean Asset::content() on OutputAssets is only called when the file is actually requested. So we want to delay as much work as possible to this point. e. g. Chunks only generate the content in the content() method, so code generation is skipped until this point.
By having RawOutput::new(polyfill_output_path, polyfill_module) the polyfill_module.content() method would only be called when the file is requested. So that would cause the polyfill file not to be read until it's actually requested from the browser.

[bgw]

Because this requires changes to turbopack_core, I'm going to leave it as VirtualOutputAsset in this PR, but I'll make a separate PR for the turbo repo to add a path to RawOutput, and a follow-up to switch this to RawOutput.

Edit: vercel/turborepo#8075 and #65300