Validate time zone in quickDT prior to guessing offset

[schleyfox]

This is part of a series of PRs based on performance work we have done to
improve a use-case involving parsing/formatting hundreds of thousands of dates
where luxon was the bottleneck.

We validate the zone in the constructor anyway, but the tests missed a branch where we attempt to use an invalid zone to compute an offset, which will result in an exception. Zone#isValid is calculated at creation time (and checked in DateTime constructor already) so this will not degrade performance in the happy case.

We discovered this when shifting callsites to use DateTime.local from things that previously called through fromObject.

[linux-foundation-easycla]

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• ✅ login: schleyfox / name: Ben Hughes (6072eba)

[icambron]

Looks good, thanks!