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/Forward.h

@@ -32,6 +32,7 @@ class ASTNode;
 class Argument;
 class Cell;
 class Error;
+class Exception;
 class Expression;
 class Function;
 class HandleImpl;