Restore Dates.default_format(::Type{ZonedDateTime})

[omus]

Was removed in #483 as we believed this was only used with parsing. We did discover that there were some legitimate use cases of this with Dates.format: quinnj/JSON3.jl#313. I thought maybe we could modify JSON3.jl to work around this but while implementing that change I changed my mind on what was the right approach.

As of now I think that:

• Most users should avoid using Dates.default_format for parsing as they won't be able to parse ZonedDateTime strings without milliseconds. They should use parse(ZonedDateTime, str) instead.
• When using Date.format it is reasonable to use Dates.default_format(::Type{ZonedDateTime}).

[omus]

Nightly failure for x86 is unrelated:

allocations: Test Failed at /home/runner/work/TimeZones.jl/TimeZones.jl/test/types/timezone.jl:45
  Expression: #= /home/runner/work/TimeZones.jl/TimeZones.jl/test/types/timezone.jl:45 =# @allocations(istimezone("America/Winnipeg")) == 1
   Evaluated: 0 == 1