Add RelativedeltaConverter and parse_relativedelta (#5000)

* Added years and months to parse_timedelta

* Added new parse_datetimedelta along with classes for relative dates

* Switched datetime as dt to just datetime for clarity

* Changed to returning relativedelta instead of datetime

* Fixed single char typo

* After some digging, removed min and max from relative delta b/c of https://github.com/dateutil/dateutil/issues/350

* Add dateutil to intersphinx mapping

* Change uppercase D in RelativeDeltaConverter to a lowercase D

* Fix cross-references in docstrings

* Add new class and methods to __all__

* Remove get_relativedelta_converter()

* style

* Fix name of parse_relativedelta test

* more style

* Re-export new class and function in `redbot.core.commands`

Co-authored-by: jack1142 <6032823+jack1142@users.noreply.github.com>

docs/conf.py

@@ -218,6 +218,7 @@ intersphinx_mapping = {
     "dpy": (f"https://discordpy.readthedocs.io/en/v{dpy_version}/", None),
     "motor": ("https://motor.readthedocs.io/en/stable/", None),
     "babel": ("http://babel.pocoo.org/en/stable/", None),
+    "dateutil": ("https://dateutil.readthedocs.io/en/stable/", None),
 }
 
 # Extlinks