Overhauled the Welsh formats.py file.

The only debatable thing here is setting FIRST_DAY_OF_WEEK to 1.
The only examples of calendars I can find in Welsh start on Monday,
for example, the calendar on S4C (the only all Welsh TV station).

http://www.s4c.co.uk/clic/c_clicschedule.shtml

Monday is also the first day of the week in modern en_GB usage,
though the formats.py file there doesn't yet reflect this.

For issue #23368.