LibJS: Implement Array.prototype.flatMap

Also made recursive_array_flat more compliant with the spec
So renamed it to flatten_into_array

Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.flatMap.js

@@ -0,0 +1,71 @@
+test("length is 1", () => {
+    expect(Array.prototype.flatMap).toHaveLength(1);
+});
+
+describe("normal behavior", () => {
+    test("basic functionality", () => {
+        function identity(i) {
+            return i;
+        }
+
+        var array1 = [1, 2, [3, 4]];
+        var array2 = [1, 2, [3, 4, [5, 6]]];
+        expect(array1.flatMap(identity)).toEqual([1, 2, 3, 4]);
+        // only goes to depth 1
+        expect(array2.flatMap(identity)).toEqual([1, 2, 3, 4, [5, 6]]);
+    });
+
+    test("flattens return values", () => {
+        function double(i) {
+            return [i, 2 * i];
+        }
+
+        var array1 = [1, 2];
+        var array2 = [1, [3]];
+        expect(array1.flatMap(double)).toEqual([1, 2, 2, 4]);
+
+        // looks weird but it is correct
+        expect(array2.flatMap(double)).toEqual([1, 2, [3], 6]);
+    });
+
+    test("binds this value", () => {
+        let this_ = undefined;
+        function callable() {
+            this_ = this;
+        }
+        const this_arg = { "yak?": "always" };
+        [0].flatMap(callable, this_arg);
+        expect(this_).toEqual(this_arg);
+    });
+
+    test("gives secondary arguments", () => {
+        const found_values = [];
+        const found_indices = [];
+        const found_array_values = [];
+        const found_this_values = [];
+        function callable(val, index, obj) {
+            found_values.push(val);
+            found_indices.push(index);
+            found_array_values.push(obj);
+            found_this_values.push(this);
+        }
+        const this_arg = { "yak?": "always" };
+        const array = ["a", "b", "c"];
+        array.flatMap(callable, this_arg);
+
+        expect(found_values).toEqual(["a", "b", "c"]);
+        expect(found_indices).toEqual([0, 1, 2]);
+        expect(found_array_values).toEqual([array, array, array]);
+        expect(found_this_values).toEqual([this_arg, this_arg, this_arg]);
+    });
+
+    test("empty array means no calls", () => {
+        let called = false;
+        function callable() {
+            called = true;
+            throw "Should not be called";
+        }
+        [].flatMap(callable);
+        expect(called).toBeFalse();
+    });
+});