Use ISO standard for day numbering

[WickyNilliams]

Right now I use the JS 0-indexed approach for day numbering throughout the codebase e.g.

• Sunday 0
• Monday 1
• etc

This is used consistently throughout cally:

• firstDayOfWeek defaults to 1 but is 0-indexed
• CSS parts for day numbers e.g. day-0

But since the goal is to move entirely to Temporal API when it's available, we should switch to the ISO standard of 1-indexed day numbering.

Unfortunately this is a breaking change, so I will likely save for v1

[SalahAdDin]

@WickyNilliams How is this going on?

[WickyNilliams]

It is likely going to be part of the v1 release. I don't have strong plans for when that will be yet, but it's definitely something I'm close to in terms of feature set