ENH: Implement IntervalIndex.is_overlapping
- [X] closes #23309
- [X] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] whatsnew entry

This is needed in the `get_indexer` implementation for the new `IntervalIndex` behavior, as an overlapping `IntervalIndex` may return non-unique indices for a given query; seems cleaner to implement separately. Also makes sense as a general attribute for an `IntervalIndex` to have.