Class `DocumentRange` belongs in rtextarea package, not rsyntaxtextarea

[bobbylight]

Description
The org.fife.ui.rsyntaxtextarea.DocumentRange class should be in the sister rtextarea package. It might live in the wrong place due to a refactoring a while ago.

Note this doesn't impact the library's functionality in any way, but since it is a breaking API change, it requires a major version bump.

Steps to Reproduce
No reproduction steps; this is an API design issue.

Expected behavior
N/A

Actual behavior
N/A

Screenshots
N/A

Java version
N/A

Additional context
This class is used in classes in the sister rtextarea package, so we have a circular dependency here.