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/Parser.h

@@ -52,6 +52,7 @@ public:
     NonnullRefPtr<VariableDeclaration> parse_variable_declaration();
     NonnullRefPtr<ForStatement> parse_for_statement();
     NonnullRefPtr<IfStatement> parse_if_statement();
+    NonnullRefPtr<ThrowStatement> parse_throw_statement();
     NonnullRefPtr<TryStatement> parse_try_statement();
     NonnullRefPtr<CatchClause> parse_catch_clause();