Add spell check (#3544)

I felt like adding a spell checker, but it's okay if you find it
superfluous. At least then we could fix the found spell issues.

What is still missing is an automatic integration so that the spell
checker does not have to be called manually. Would it perhaps make sense
to always do it before a release?

tests/e2e/modules_display_spec.js

@@ -12,7 +12,7 @@ describe("Display of modules", () => {
 	it("should show the test header", async () => {
 		const elem = await helpers.waitForElement("#module_0_helloworld .module-header");
 		expect(elem).not.toBeNull();
-		// textContent gibt hier lowercase zurück, das uppercase wird durch css realisiert, was daher nicht in textContent landet
+		// textContent returns lowercase here, the uppercase is realized by CSS, which therefore does not end up in textContent
 		expect(elem.textContent).toBe("test_header");
 	});