proposal: Add an option to allow deploying into the main branch

[alvinsjoy]

Checklist

• I am using the latest version of this action.
• I have read the latest README and followed the instructions.
• I have read the latest GitHub Actions official documentation and learned the basic spec and concepts.

Describe your proposal

As mentioned in #363, one cannot deploy into the main (or master) branch of the repository as all the contents would be deleted but as you can now deploy to a sub directory, I dont think there is a problem if you are publishing to the main branch itself.
Also keep-files option can be used so that the contents on the default branch are not deleted.

Describe the solution you'd like

Allow changing publish_branch to main or master (ie. default branch)

Describe alternatives you've considered

No response

Additional context

This is the current error: