Add the ability to generate migration files with a specific name

[elia]

This pull request introduces a new command for generating Shopify migration files, updates the documentation to reflect this change, and enhances the ShopifyToolkit::CommandLine class by including additional functionality from the Thor library.

Documentation updates:

• README.md: Updated the instructions for creating a migration file to use the new shopify-toolkit generate migration command instead of manually creating the file.

New feature:

• lib/shopify_toolkit/command_line.rb: Added a new generate_migration command to the ShopifyToolkit::CommandLine class. This command generates a migration file with a timestamped filename and a basic class structure for migrations.

Code enhancements:

• lib/shopify_toolkit/command_line.rb: Included Thor::Actions in the ShopifyToolkit::CommandLine class to enable file generation and other utility methods provided by Thor.