Support getting ISO week date, week of year

There are some simple date functions available in Bicep: https://learn.microsoft.com/en-us/azure/azure-resource-manager/bicep/bicep-functions-date

With https://learn.microsoft.com/en-us/dotnet/api/system.globalization.isoweek?view=net-8.0, we can get the standardized week day formats in c#, without having to bother with localization.

Useful in
- a sandbox environment where soft delete resources (like key vaults) are cleaned up automatically (but not purged), thus needing a resource name with reliable increment postfix which doesn't change too often, but have to be unique enough to not collide with existing soft deleted resources (thus weeknumbers are perfect, at least in my case)
- tagging strategy
- description fields

Start by reviewing Bicep's existing date functions and the linked .NET ISOWeek API to compare the requested standardized week-date and week-of-year behavior. Define the expected results and edge cases from ISO week-date rules, then verify that the new capability is documented and covered by appropriate tests.

Written by the indexing model from the issue text.