fix(nuxt): usePinia composable type

[th1m0]

This PR fixes a type issue in the nuxt module. using the composable usePinia would always return unknown while it should always return the Pinia type.

Before: composable.d.ts

export * from 'pinia';
export declare const usePinia: () => unknown;

After: composable.d.ts

import type { Pinia } from 'pinia';
export * from 'pinia';
export declare const usePinia: () => Pinia;

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