based on https://en.wikipedia.org/wiki/ISO_week_date#Calculating_the_week_number_of_a_given_date
M / Power Query doesn't have a native ISO8601 Week Number function, and DAX's
weeknum(x, 21)
doesn't give the correct ISO Week-Year.
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based on <https://en.wikipedia.org/wiki/ISO_week_date#Calculating_the_week_number_of_a_given_date>