LibJS: Support spreading of strings and string objects

Libraries/LibJS/Tests/string-spread.js

@@ -0,0 +1,31 @@
+load("test-common.js");
+
+function testArray(arr) {
+    return arr.length === 4 &&
+        arr[0] === "a" &&
+        arr[1] === "b" &&
+        arr[2] === "c" &&
+        arr[3] === "d";
+}
+
+try {
+    var arr;
+
+    arr = ["a", ..."bc", "d"];
+    assert(testArray(arr));
+
+    let s = "bc";
+    arr = ["a", ...s, "d"];
+    assert(testArray(arr));
+
+    let obj = { a: "bc" };
+    arr = ["a", ...obj.a, "d"];
+    assert(testArray(arr));
+
+    arr = [..."", ...[...new String("abc")], "d"];
+    assert(testArray(arr));
+
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}