Add dates() utility function for explicit year-to-date mappings

[lauft]

Introduce a new dates() function that provides a cleaner way to define holidays with explicit year-to-date mappings, replacing static methods in region classes.

This simplifies holiday definitions by allowing inline date mappings with optional fallback to dynamic date calculations via or_else_on().

Example:

self.define_holiday() \
    .with_name("Grand Final Eve") \
    .since(2019) \
    .on(dates({
        2020: (Month.OCTOBER, 23),  # Adaption due to the COVID-19 pandemic
    }).or_else_on(first(Weekday.FRIDAY).before(last(Weekday.SATURDAY).of(Month.SEPTEMBER)))) \
    .with_flags("V")