Allow grammar rule customization for unit declensions

[kylekatarnls]

This will allow declension in Mongolian to apply only to the last part when the interval is a sequence of units:

When last unit can be shorten, we just get no declensions:

Carbon::now()
    ->subYears(2)
    ->subMonths(3)
    ->subDay()
    ->subSecond()
    ->locale('mn')
    ->diffForHumans(null, null, true, 4)

2 жил 3 сар 1 өдөр 1с өмнө

When last unit is entire, we get declension only on this last unit:

Carbon::now()
    ->subYears(2)
    ->subMonths(3)
    ->subDay()
    ->subSecond()
    ->locale('mn')
    ->diffForHumans(null, null, false, 4)

2 жил 3 сар 1 өдөр 1 секундын өмнө

Fix #2587
Fix #2565

👋 @lucifer-crybaby @Batmandakh your review is welcome. 🙏