Shell: Use NonnullRefPtr to simplify some things in the parser/AST

Shell/Parser.cpp

@@ -72,7 +72,7 @@ bool Parser::expect(const StringView& expected)
 }
 
 template<typename A, typename... Args>
-RefPtr<A> Parser::create(Args... args)
+NonnullRefPtr<A> Parser::create(Args... args)
 {
     return adopt(*new A(AST::Position { m_rule_start_offsets.last(), m_offset }, args...));
 }