LibJS: Make Function.prototype a callable function object

20.2.3 Properties of the Function Prototype Object
https://tc39.es/ecma262/#sec-properties-of-the-function-prototype-object

The Function prototype object:
- is itself a built-in function object.

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

@@ -0,0 +1,26 @@
+describe("correct behavior", () => {
+    test("length is 0", () => {
+        expect(Function.prototype).toHaveLength(0);
+    });
+
+    test("name is empty string", () => {
+        expect(Function.prototype.name).toBe("");
+    });
+
+    test("basic functionality", () => {
+        function fn() {}
+        expect(Object.getPrototypeOf(fn)).toBe(Function.prototype);
+        expect(Object.getPrototypeOf(Function.prototype)).toBe(Object.prototype);
+    });
+
+    test("is callable", () => {
+        expect(Function.prototype()).toBeUndefined();
+    });
+
+    test("is not constructable", () => {
+        expect(() => new Function.prototype()).toThrowWithMessage(
+            TypeError,
+            "[object FunctionPrototype] is not a constructor (evaluated from 'Function.prototype')"
+        );
+    });
+});