Feature request: support milliseconds in isDate

[sargunv]

I'm trying to parse JSON that contains dates that are just like the ISO8601 handled by this library, except they have milliseconds. I get the following error from Typanion:

Expected to be a valid ISO 8601 date string (got "2023-01-06T02:10:20.118Z")

I'd like either the existing isDate or a new function to support the .118 milliseconds part of the string.

[ztamas83]

@arcanis I created a quick PR, would highly appreciate if a new release could be created with the added regex. It's not complete as per #39, but would help me a lot in an ongoing project.