add type declaration for vitePreprocess reexport

[gtm-nayan]

aside: Using 'declare module' for these exports kind of feels like a hack to me, unless it's needed for the docs we should maybe be using "exports.types" alongside the js files listed in package.json but out of scope for this PR.

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[Rich-Harris]

Using 'declare module' for these exports kind of feels like a hack to me

Agree with that sentiment, I can't remember if there's a reason we're doing in this way rather than generating .d.ts files in prepublishOnly. (Though writing them by hand is useful for one very specific reason — we can rename a function like invalid to fail in a way that will cause a red squiggly when you import the old function but that will also allow us to export a function with the old name that errors at runtime with a helpful error.)