feat: enable support for exports condition

[dominikg]

This PR enables using an export condition svelte to point at svelte source files.

eg this exports map

	"exports": {
		"./*": {
			"svelte": "./src/components/*"
		},
		".": {
			"svelte": "./index.js"
		}
	}

allows you to either import {SomeThing} from 'some-lib' when index.js exports SomeThing, or import SomeThing from 'some-lib/SomeThing.svelte' if you prefer a deep import to avoid the index.

This is a very simplistic example and the real usecase is to still allow exporting compiled svelte files or other js files via the import condition.

See https://nodejs.org/api/packages.html#conditional-exports for what you can do with export conditions.

[dominikg]

do we need additional tests, eg a scenario with mixes svelte field and exports condition? Is the usage i described above what we want to go for?
i tried using the .svelte extension with glob but it didn't work even if the node docs show tha *.js is a thing.

	"exports": {
	       // boohoo, didn't work in my tests
		"./*.svelte": {
			"svelte": "./src/components/*.svelte"
		},
		".": {
			"svelte": "./index.js"
		}
	}

We only ever expect libraries to use the .svelte extension with preprocessed .svelte output, never other extensions like .svx that require additional markup preprocessors to be present, correct?

[benmccann]

I wonder if it's just *.svelte that doesn't work or if other extensions are affected. My guess is that it's an existing Vite bug, but it might be nice to confirm and file an issue.

[bluwy]

i tried using the .svelte extension with glob but it didn't work even if the node docs show tha *.js is a thing.

Yeah I think this should technically work too, hoping it's only a bug on not some hidden nodejs spec.

[bluwy]

one nit but lgtm!