Modernization & support for migration / items models

[Enngage]

Sum of the PR:

• Complete overhaul of the repo code & modernization
• Support for migration models (https://www.npmjs.com/package/@kontent-ai/migration-toolkit)
• Support for item models (aka strongly typed access to item codenames / ids) - https://github.com/kontent-ai/model-generator-js/issues/32
• Improved JS Delivery models (added support for typed system props)