[SocialNetwork] Support additional userSource values

[beefchimi]

Currently, most social networks define their user handle in the path. Typically, it is the first segment, but occasionally it is proceeded by a directory such as /profiles/ or /users/.

There is at least one supported network that defines the user handle as a subdomain. substack has urls like: https://username.substack.com/.

To solve this, I introduced a userSource property to the SocialNetwork > SocialNetworkMatcher interface that will allow developers to specify either MatchUserSource.Subdomain | MatchUserSource.Path, with path being the default behaviour within socialite.ts. This isn't the most eloquent solution... but it works for now.

We may find some future social networks supporting different user handle definitions, such as in the parameters. If ever we have a reason to support this, we will need to either:

1. Add another option to MatchUserSource
2. Find a better solution to userSource