Implement method `CppStringT::rstrip()`

[schmouk]

"str.rstrip([chars])

Return a copy of the string with trailing characters removed. The chars argument is a string specifying the set of characters to be removed. If omitted or None, the chars argument defaults to removing whitespace. The chars argument is not a suffix; rather, all combinations of its values are stripped.

See str.removesuffix() for a method that will remove a single suffix string rather than all of a set of characters."

[schmouk]

Completed.