Enhanced the way of using locales in format function

[devmuaz]

This PR should make using the locales easier, by pre-defining them by default statically in that the user won't make a mistake writing the wrong string locale.

So, instead of:

	final ago = format(date, locale: 'es');
	// or
	final agoShort = format(date, locale: 'es_short');

You can use this:

	final ago = format(date, locale: DefaultLocale.es);
	// or
	final agoShort = format(date, locale: DefaultLocaleShort.es);