fix: adjust app.d.ts to diminish confusion about imports

[dummdidumm]

closes #8276

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[Rich-Harris]

Pleasantly surprised to discover that export declare works — solves the problem without export default undefined which is fugly and requires explanation. Unless there's a reason not to use export declare?

[dummdidumm]

As discussed offline, export declare global is unfortunately a TS error, so is export default {} instead of export default undefined (which admittedly looks weird, but it's the shortest way to get TS to play ball).