Make use of @jest/globals

This makes it easier for TypeScript to typecheck our tests, rather
relying on `any`-typed globals.

For more details, see
https://jestjs.io/blog/2020/05/05/jest-26#a-new-way-to-consume-jest---jestglobals

Unfortunately, it was not possible to
`import { jest } from "@jest/globals"` in files that also call
jest.mock(). This was reported here:
jestjs/jest#10060

src/datatypes.test.ts

@@ -1,3 +1,4 @@
+import { describe, it, expect } from "@jest/globals";
 import { DataFactory } from "n3";
 import { LocalNode } from "./index";
 import {

src/fetcher.test.ts

@@ -1,9 +1,13 @@
+import { it, expect } from "@jest/globals";
 jest.mock("cross-fetch");
 
 import { fetch } from "./fetcher";
 
 it("should fallback to cross-fetch if no Solid-specific fetcher is available", async () => {
-  const crossFetch = jest.requireMock("cross-fetch");
+  const crossFetch = jest.requireMock("cross-fetch") as jest.Mock<
+    ReturnType<typeof window.fetch>,
+    [RequestInfo, RequestInit]
+  >;
 
   await fetch("https://some.url");