Load fetch types are wrong

[ebeloded]

This is a regression introduced in #670

LoadInput's fetch now returns Response imported from types.d.ts, which is not Fetch API response.

// types-internal.d.ts
import { ..., Response } from './types'; // <- problem
export type LoadInput = {
	page: Page;
	fetch: (info: RequestInfo, init?: RequestInit) => Promise<Response>; // <- wrong response
	session: any;
	context: Record<string, any>;
};

// types.d.ts
export type Response = {
	status?: number;
	headers?: Headers;
	body?: any;
};

PS. To catch type-related issues, maybe transition examples to TypeScript and use svelte-check to verify correctness?