Add Phone Number Linking

[lukenewman]

This PR adds PhoneNumberLinkifier (phone_number.dart) that parses phone numbers and allows opening of phone number URLs (tel:##########).

[lukenewman]

I have tested this on iOS but not on Android. I have no physical Android device to test this on.

[Cretezy]

Sorry I haven't been able to address this sooner.

Could you please add tests? It's unclear from the regex what type of phone numbers (they are multiple formats internationally) are support from this. Additionally, adding some docs in the README would be useful. I wouldn't activate this as a default linkifier as it may parse non-phone numbers