Improve phone number validation

[rimas-kudelis]

This fixes #820 in a more complete way.

[alextselegidis]

Great thanks, this looks good!

Alex Tselegidis, Easy!Appointments Creator Need a customization? Get a free quote!

[rimas-kudelis]

Potential caveats/improvement options here:

• I only include libphonenumber-js in the customer "create appointment" UI, so the thorough front-end check will not happen in other cases (it will fall back to your current logic).
• Same applies to backend validation: I only perform it when a customer is creating an appointment, not in any other case.
• Also, the setting only affects backend.
• Unless you specify a specific country to use by default, both frontend and backend validations are country-agnostic, meaning, they will require a phone in full international +123456789 format. It's possible to pass a country code to the validation, in which case numbers for that country will be accepted in locally standardised prefix (e.g. 0123456789).