[Feature Request] Week number option in calendar popup

[sfc-gh-afedorov]

Itsycal has a feature where the week number is displayed next to the calendar, which is really useful because a lot of my work planning and operations breaks reports and metrics down by week number in the year.

To this end, please add an option to add the ISO week number as a left-hand column to the calendar popup UI.

An additional feature others might want [1] is to use week 1 always starting on January 1st, which is not something Itsycal currently intends to support.

1. Calendar weeks do not match unicode week numbers sfsam/Itsycal#183