Add check that abstract rules belong to the same pattern as extends instructions referring to them

[AndrewSales]

Re schematron-quickfix/sqf#24, the text quoted by @octavianN -

"An abstract rule is a list of assertions that will be invoked by other rules belonging to the same pattern using the extends element." [my emphasis]

has not changed in the 2020 edition, and the current schematron.sch does not check this constraint.