This requires the package miniflare (NOT any of the @miniflare/-scoped packages), as well as @cloudflare/workers-types if you're using TypeScript.
For setting up @sveltejs/adapter-cloudflare, see https://kit.svelte.dev/docs/adapter-cloudflare
// src/hooks.server.ts
import { dev } from "$app/environment";
import type { Miniflare } from "miniflare";