ENH: Implement MultiIndex.is_monotonic_decreasing
- [X] closes #16554
- [X] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] whatsnew entry

Note that this is a dupe PR of #16573, which appears to have gone stale.

Implementation ended up being easier than what I suggested in the issue; can use the fact that an index is monotonic decreasing if it's reverse is monotonic increasing.  Just a matter of reversing the index and calling `is_monotonic_increasing`.

Regarding `MultiIndex` tests:
- Added a test case to the `is_monotonic_increasing`  test to verify it's working for an empty index.
- The `is_monotonic_decreasing` test cases consist of decreasing versions of the test cases in the `is_monotonic_increasing` test.

Regarding `IntervalIndex` tests:
- Added a test for `IntervalIndex.is_monotonic_increasing` and `IntervalIndex.is_monotonic_decreasing`  since they use the MultiIndex implementation under the hood.
- Split a test that originally checked both monotonic and `is_unique` into separate tests.  Expanded the test cases for both tests.