feature: add user model & migration

[notmarkmiranda]

Closes #3

Description

This PR introduces several new features and improvements:

1. User Model & Migration:

   • Adds a User model with the necessary database migration.
   • Includes validations for the phone_number attribute using the Phonelib gem.

2. Validations:

   • Ensures phone_number is present, unique, and correctly formatted.

3. Faker Gem Integration:

   • Introduces the Faker gem for generating test data in factory_bot.
   • Faker will also be utilized in production to auto-generate nicknames.

Changes

• Gemfile: Added phonelib and faker gems.
• Gemfile.lock: Updated to include the new gem dependencies.
• app/models/user.rb: Created the User model with the necessary validations and a before_validation callback to format the phone number.
• config/initializers/phonelib.rb: Set the default country for Phonelib to "US".
• db/migrate/20240616032726_create_users.rb: Migration file to create the users table with the required fields.
• db/schema.rb: Updated schema to include the users table.
• spec/factories/users.rb: FactoryBot definition for the User model.
• spec/models/user_spec.rb: RSpec tests for the User model to ensure phone number validation and formatting.