Check the regex that matches subdomain and path

[giac-mysten]

Ensure that:

• only one level of subdomain nesting is allowed
• any valid path is supported
• ...

[jpcsmith]

This can make use of new URL(..) to do the splitting instead of a regex, and count the length of the resulting vector when splitting the domain.

Would need to find a way to handle urls ending in things like co.uk.