LibJS: Implement indexed access for StringObject

Author: https://github.com/linusg
Commit: 1ba2e6768d
Pull-request: https://github.com/SerenityOS/serenity/pull/2047
Reviewed-by: https://github.com/awesomekling

Libraries/LibJS/Tests/indexed-access-string-object.js

@@ -0,0 +1,19 @@
+load("test-common.js");
+
+try {
+    var s = "foo";
+    assert(s[0] === "f");
+    assert(s[1] === "o");
+    assert(s[2] === "o");
+    assert(s[3] === undefined);
+
+    var o = new String("bar");
+    assert(o[0] === "b");
+    assert(o[1] === "a");
+    assert(o[2] === "r");
+    assert(o[3] === undefined);
+
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}