Add Week Number

[sindresorhus]

Types of Changes

• [x] New addition to list
• [ ] Fixing bug in existing item in list
• [ ] Removing item from list
• [ ] Changing structure (organization) of list

Proposed Changes

Needing to know the week number is a common need in Europe where it's used a lot. This puts the week number front and center, and also lets you quickly find the week number for a specific date.

https://sindresorhus.com/week-number

PR Checklist

• [x] I have read the CONTRIBUTING guide
• [x] I have explained why this PR is important
• [x] I have added necessary documentation (if appropriate)