LibJS: Implement Error.prototype.toString()

Author: https://github.com/linusg
Commit: 94afcb54de
Pull-request: https://github.com/SerenityOS/serenity/pull/1590

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

@@ -0,0 +1,13 @@
+function assert(x) { if (!x) throw 1; }
+
+try {
+    assert(Error().toString() === "Error");
+    assert(Error(undefined).toString() === "Error");
+    assert(Error(null).toString() === "Error: null");
+    assert(Error("test").toString() === "Error: test");
+    assert(Error(42).toString() === "Error: 42");
+
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}