Support comparing DateDifference and Duration

[evansd]

It ought to be possible to write things like:

where((some_date - another_date) < days(28))

which should compile to the same thing as:

where((some_date - another_date).days < 28)

The relevant classes here are:

ehrql/ehrql/query_language.py

Lines 778 to 782 in f704c11

	class DateDifference:
	"""
	Represents the difference between two date series (i.e. it is what you get when you
	subtract one date series from another)
	"""

ehrql/ehrql/query_language.py

Lines 819 to 820 in f704c11

	class Duration:
	value: int | IntEventSeries | IntPatientSeries