Add support for yMMdd dateformat

[aanelson]

Is your feature request related to a problem? Please describe.

Sometimes dates need a leading zero in the day/month field and the only way to do this is with a custom DateFormat.
For example
final dateFormat = DateFormat('MM/dd/yyyy', locale.toLanguageTag());

However, localization does not work correctly if you use a custom date format.

Describe the solution you'd like

Add DateFormat.yMMdd()
Add add_MM() and add_dd() methods.

Describe alternatives you've considered

static extension on DateFormat for yMMdd, however would have to handle locale mapping.

[mosuem]

While yMd exists in the CLDR data, yMMd does not. So this is not a pattern this package can support.