Rename parameters to be consistent with JamesIves' action

It was a silly idea to make this action so parallel to JamesIves', while also renaming parameters.

E.g. 'source-dir' should be renamed to 'folder'.

This will result in much easier integration with projects using JamesIves/github-pages-deploy-action, which is the intended use case.

I should retain the current parameters, but mark them as deprecated.