### Description

Currently, the `airflow variables export` command requires an explicit file path and does not support output to stdout. However connections can be printed to stdout using `airflow connections export -`. This inconsistency between the two export commands can lead to confusion and limits the flexibility of the variables export command.

### Use case/motivation

To bring some consistency, similar to connections, variables should also be printed to STDOUT, using `-` instead of filename. 

### Related issues

_No response_

### Are you willing to submit a PR?

- [X] Yes I am willing to submit a PR!

### Code of Conduct

- [X] I agree to follow this project's [Code of Conduct](https://github.com/apache/airflow/blob/main/CODE_OF_CONDUCT.md)

"""CustomParser to display help message."""
    def _check_value(self, action, value):
        """Override _check_value and check conditionally added command."""
        if action.choices is not None and value not in action.choices:
            check_legacy_command(action, value)
        super()._check_value(action, value)
    def error(self, message):
        """Override error and use print_instead of print_usage."""
        self.print_help()
        self.exit(2, f"\n{self.prog} command error: {message}, see help above.\n")
_UNSET = object()
class Arg:
    """Class to keep information about command line argument."""
    def __init__(