Add --days, --weeks, --months arguments for relative date ranges

[Niels-R]

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Changes proposed in this Pull Request:

Allows generating orders within a relative time period (e.g. --days=21, --weeks=4, --months=6) without needing to compute explicit calendar dates. The period arguments resolve to a date-start/date-end pair internally, reusing all existing date logic. Adds CLI validation and five new unit tests.

How to test the changes in this Pull Request:

1. Generate orders using each new argument and verify the created dates fall within the expected range in WooCommerce > Orders:
   wp wc generate orders 10 --days=14
wp wc generate orders 10 --weeks=4
wp wc generate orders 10 --months=2

2. Verify that combining a period argument with --date-start or --date-end produces an error:
   wp wc generate orders 10 --days=7 --date-start=2025-01-01

3. Verify that passing multiple period arguments or an invalid value produces an error:
   wp wc generate orders 10 --days=7 --weeks=2
wp wc generate orders 10 --months=0

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully run tests with your changes locally?

Changelog entry

Add --days, --weeks, and --months arguments to wp wc generate orders, allowing orders to be generated within a relative time period from today without needing to specify explicit calendar dates.

FOR PR REVIEWER ONLY:

• I have reviewed that everything is sanitized/escaped appropriately for any SQL or XSS injection possibilities. I made sure Linting is not ignored or disabled.