LibJS: Implement Function.prototype.toString() according to the spec

That's an old yak :^)
No, past me, AST nodes do not need to learn to stringify themselves.
This is now massively simplified by using the [[SourceText]] internal
slot.

Also updates a bunch of tests that are incorrect due to the old
implementation not being spec compliant, and add plenty more.

Userland/Libraries/LibJS/Tests/builtins/Function/Function.js

@@ -33,7 +33,7 @@ describe("correct behavior", () => {
         expect(new Function("-->")()).toBeUndefined();
 
         expect(new Function().name).toBe("anonymous");
-        expect(new Function().toString()).toBe("function anonymous() {\n  ???\n}");
+        expect(new Function().toString()).toBe("function anonymous(\n) {\n\n}");
     });
 });