Implement -v flag

I want to add support for the `-v`(`--variables`) operation that currently exists in the gettext version.
more info [here](https://www.gnu.org/software/gettext/manual/html_node/envsubst-Invocation.html)

The issue references the gettext manual for the -v/--variables flag behavior. Look at the existing command-line parsing and substitution logic in the codebase, likely in main.go or a cmd/ package. Understand how variables are currently extracted and implement the flag to list them without substitution. Check for existing tests to see the expected output format.

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