LibJS: Add String.prototype.toString()

Author: https://github.com/linusg
Commit: 32c27afdf0
Pull-request: https://github.com/SerenityOS/serenity/pull/1729

Libraries/LibJS/Tests/String.prototype.toString.js

@@ -0,0 +1,9 @@
+try {
+    assert(String.prototype.toString.length === 0)
+    assert("".toString() === "");
+    assert("hello friends".toString() === "hello friends");
+
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}