Let ArgumentDescription#getDescription and #isEmpty have command sender

[egg82]

Currently, ArgumentDescriptions have no way to tell who is requesting the description.

This is a useful feature for sending different descriptions depending on the person requesting them. For example, a custom localization manager would require the requesting user to be known (for platforms without Adventure)

This PR generifies ArgumentDescription and Description and adds a C sender argument to the getDescription and isEmpty methods.

Potential downsides of this would be the lack of backwards compatibility, but because the signatures have changed compiler errors will be issued rather than runtime exceptions.

Seems to compile fine with ./gradlew clean build