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.

Libraries/LibJS/Interpreter.cpp

@@ -188,7 +188,7 @@ Value Interpreter::call(Function* function, Value this_value, const Vector<Value
     return result;
 }
 
-Value Interpreter::throw_exception(Error* exception)
+Value Interpreter::throw_exception(Exception* exception)
 {
     m_exception = exception;
     unwind(ScopeType::Try);