reader: Add delete-or-exit bind function

This is an important binding and should therefore be in C++.

src/input.cpp

@@ -132,6 +132,7 @@ static const input_function_metadata_t input_function_metadata[] = {
     {readline_cmd_t::reverse_repeat_jump, L"repeat-jump-reverse"},
     {readline_cmd_t::func_and, L"and"},
     {readline_cmd_t::expand_abbr, L"expand-abbr"},
+    {readline_cmd_t::delete_or_exit, L"delete-or-exit"},
     {readline_cmd_t::cancel, L"cancel"}};
 
 static_assert(sizeof(input_function_metadata) / sizeof(input_function_metadata[0]) ==