LibJS: Implement basic for..in and for..of loops

Libraries/LibJS/Tests/for-of-basic.js

@@ -0,0 +1,43 @@
+load("test-common.js");
+
+try {
+    assertThrowsError(() => {
+        for (const _ of 123) {}
+    }, {
+        error: TypeError,
+        message: "for..of right-hand side must be iterable"
+    });
+
+    assertThrowsError(() => {
+        for (const _ of {foo: 1, bar: 2}) {}
+    }, {
+        error: TypeError,
+        message: "for..of right-hand side must be iterable"
+    });
+
+    assertVisitsAll(visit => {
+        for (const num of [1, 2, 3]) {
+            visit(num);
+        }
+    }, [1, 2, 3]);
+
+    assertVisitsAll(visit => {
+        for (const char of "hello") {
+            visit(char);
+        }
+    }, ["h", "e", "l", "l", "o"]);
+
+    assertVisitsAll(visit => {
+        for (const char of new String("hello")) {
+            visit(char);
+        }
+    }, ["h", "e", "l", "l", "o"]);
+
+    var char;
+    for (char of "abc");
+    assert(char === "c");
+
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}