allow format to be used according to the current locale

[luiz-soares]

Each Locale has a default regarding the usage of periods when defining times (AM/PM).

While some might play nice with both 12h or 24h hours format, a default still exists and is the most common for that locale, and some cases

It would be nice if we could leave format input empty and go with the Locale default regarding periods.

I understand this would be a breaking change, so perhaps we start keeping as 12 but allowing it to be set to null/undefined and then do not override its property when asking luxon to do the formatting.

I can work on this, let me know if you're ok with this approach.

[Agranom]

Hi. You can work on it