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Add monotonic_year and tests (#6800)

Supersedes #6762

This also fixes the bug that we were bounds checking extended year: You
should be able to do things like `Gregorian::new(year = -100, era =
None, ...)`, but we errored here.


In the process; I discovered that Chinese was _accepting_ related_iso as
extended year, but _producing_ ICU4C extended year. This is
inconsistent.


This serves as another nail in the coffin for any ICU4C-parity guarantee
we had for extended_year, because we now have found multiple cases where
we are not handling it the ICU4C way already.

The name monotonic_year is to be bikeshed.

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Co-authored-by: Robert Bastian <4706271+robertbastian@users.noreply.github.com>

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