feat(isISO8601): Add support for years with more than 4 digits

[RobinvanderVliet]

Add support to isISO8601 for years with more than 4 digits. Fixes #2262.

Reference: https://en.wikipedia.org/wiki/ISO_8601#Years

Checklist

• [x] PR contains only changes related; no stray files, etc.
• [x] README updated (where applicable)
• [ ] Tests written (where applicable)
• [x] References provided in PR (where applicable)

[pano9000]

Hi,

thanks for the PR - however I noticed that there are no test cases for this new feature. Could you kindly add tests as well - without test cases, this cannot be accepted.

thank you!