feat: add IsISO31661Numeric decorator for country codes

[RobinvanderVliet]

Description

Add IsISO31661Numeric validation decorator for ISO 3166-1 numeric countries codes. We already have IsISO31661Alpha2 and IsISO31661Alpha3, but the one for numeric codes is missing. This pull request fixes that gap.

Checklist

• the pull request title describes what this PR does (not a vague title like Update index.md)
• the pull request targets the default branch of the repository (develop)
• the code follows the established code style of the repository
  • npm run prettier:check passes
  • npm run lint:check passes
• tests are added for the changes I made (if any source code was modified)
• documentation added or updated
• I have run the project locally and verified that there are no errors

Fixes

fixes #2656