[NcDateTimePicker]: disable using not standard formatting tokens

Currently consumers of NcDateTimePicker can use format tokens that are specific to date-fns.

Proposed change

Forbid the usage of formatting tokens through public API that are only supported by date-fns.
Only allow tokens that are valid in the Unicode Technical Standard #35.

These patterns are not in the Unicode Technical Standard #35:

• i: ISO day of week
• I: ISO week of year
• R: ISO week-numbering year
• t: seconds timestamp
• T: milliseconds timestamp
• o: ordinal number modifier
• P: long localized date
• p: long localized time

https://date-fns.org/v4.4.0/docs/format

Discard user provided format if it includes invalid tokens.

Rational

Currently NcDateTimePicker is not widely used and this change would not affect many consumers.
Before consumers start using it I would like to deprecate/disable it.
This would make migrating away from date-fns (used by default by @vuepic/vue-datepicker) possible in the future without having breaking changes that affect many consumers.

Implementation

NcDateTimePicker needs to check the provided format props for nonstandard tokens.
v8 is not affected as it does not use date-fns.

Start at the NcDateTimePicker component and trace how its public format prop is validated before date-fns formats the value. Check the nonstandard tokens listed in the issue and verify that invalid formats are discarded while v8 remains unaffected.