LibJS: Implement "throw"

You can now throw an expression to the nearest catcher! :^)

To support throwing arbitrary values, I added an Exception class that
sits as a wrapper around whatever is thrown. In the future it will be
a logical place to store a call stack.

Base/home/anon/js/throw.js

@@ -0,0 +1,5 @@
+try {
+    throw 123;
+} catch (e) {
+    console.log(e);
+}