Forms: add country code dropdown select option to phone number field

### What

I would like the form block to have the country code dropdown selector for the phone number field.

### Why

A user requested this.
Many users who fill in their phone numbers in the contact form do not add their country code.
For websites that are international, this would be an issue as the site owner is left guessing the country code.

### How

It could look something like this-

Locate the form block and its phone-number field entry point; the issue names no file or test, so first map where field options and validation are handled. Use the linked screenshot to define the selector's intended presentation, and confirm done means international users can choose a country code in the contact form.

Written by the indexing model from the issue text.