builtins: add the ones we want

Currently with `jtv/parity-push` (https://github.com/crashappsec/libcon4m/commit/43082bf135dc5569e8a4ffb93cf30d119321e46b), a partially migrated `chalk.c42spec` produces `fn_not_found` errors for:

- `attr_get`
- `ends_with`
- `high`
- `path_split`
- `sections`

and for the chalk c4m files, the `fn_not_found` errors are for:

- `$`
- `array_add`
- `command_argv`
- `command_name`
- `env`
- `env_exists`
- `error`
- `find_exe`
- `hex`
- `info`
- `mime_to_dict`
- `rand`
- `run`
- `starts_with`
- `subscribe`
- `trace`
- `url_get`
- `unsubscribe`
- `using_tty`
- `write_file`

Which exact subset of these do we want as language builtins? My understanding is not at least these:

- `command_argv`
- `command_name`
- `error`
- `info`
- `trace`
- `subscribe`
- `unsubscribe`