PhoneNumber library

[woodseowl]

Adds a library class for phone number validation and formatting utilities.

This PR does the following

• Adds PhoneNumber library class (see README.md for details)
• Introduces the idea of using the Starter Kit as a library package and documents that usage

To Review:

• Review changes to the top-level README for clarity and accuracy
• Review the PhoneNumber README for clarity and accuracy
• Review the naming applied (Contact/PhoneNumber). See Rename repo + narrow focus CD-LaravelHelpers#2 for more context on the naming.
• Code review

Notes

• The test coverage is mostly just cases that have been encountered over the years in Daily Check, but they also demonstrate usage.

[richmarisa]

All very tidy.