Add support for Day of Week for ISO weeks (in IsoWeek plugin)

**Describe the bug**
Advanced formats support ISO Week of year which is wonderful, thank you! ISO Week day of week starts at 1 (not 0), and so there is currently no way to format a data in ISO Week format (since there is no `d` specifier that begins at `1` rather than `0`).

**Expected behavior**
Please consider adding an equivalent ISO Day of week to support format strings.

Start by locating the IsoWeek plugin and the existing week-formatting tests or entry points. Determine how an ISO weekday format specifier should be exposed, then verify that it produces values from 1 through 7 for ISO weeks and that the relevant formatting tests pass.

Written by the indexing model from the issue text.