Support for Monorepos

[blakewilson]

Is your enhancement related to a problem? Please describe.
Deploying WordPress plugins to WordPress.org via a monorepo.

Describe the solution you'd like
Instead of deploying the entire branch, it would be nice if a user could alternatively specify a path to the directory which should be synced to WordPress.org. Currently, the action uses the GITHUB_WORKSPACE environment variable for the directory. Maybe this option can be overridden?

[blakewilson]

Something like what is mentioned in #16 could resolve this perfectly. I think this is an important consideration with the increasing popularity of monorepos.

[richard-muvirimi]

This is addressed by #83