Add yyyy-mm-dd validation

[shintarou37]

Feature Request

Q	A
New Feature	yes
RFC	yes
BC Break	no

Summary

I would like to propose a new validator for verifying strings in the yyyymmdd format. Currently, many applications require validation of date formats, but there is a lack of support for specific formats. This new feature would allow developers to easily check if a date string conforms to a specific format. This can be incredibly useful for form input validation and data preprocessing.