ENH: Make RangeIndex.append() return RangeIndex when possible
 - [x] closes #16212
 - [x] tests added / passed
 - [x] passes ``git diff master --name-only -- '*.py' | flake8 --diff``
 - [x] whatsnew entry

This is analogous to what is already done for ``RangeIndex.union()``, but there doesn't seem to be much scope for code reuse (what could be, in principle, reused in ``.append`` of all ``Index`` subclasses is the initial casting to list/name handling, I could do in a separate PR).