feature: add `IsDurationString` to validate duration strings

Description

I would like to have an IsDurationString decorator that validates duration strings ("1 Day", "2 weeks", "7 hrs"...) from the ms package, exactly the ms.StringValue type.

An IsDuration decorator could be considered too, which validates for either a positive integer (milliseconds) or a duration string. (basically, the value is valid to pass to the ms function).

Proposed solution

Implementation of the validator could be either:

• Having ms library as a dependency: try converting the value through ms, if it succeeds then the value is valid.
• Implementing an ms-like parser for validation, without adding a dependency.