refactor: move database layer to its own package 📦

[ramiAbdou]

Description ✏️

NOTE: If you already have the repository locally, then after you merge this into your branch, you'll need to move packages/core/.env to packages/db/.env!

This PR does the following:

• Moves the database layer, including all migrations to the db package.
• Updates CONTRIBUTING.md documentation to update environment variables.

Type of Change 🐞

• Feature - A non-breaking change which adds functionality.
• Fix - A non-breaking change which fixes an issue.
• Refactor - A change that neither fixes a bug nor adds a feature.
• Documentation - A change only to in-code or markdown documentation.
• Tests - A change that adds missing unit/integration tests.
• Chore - A change that is likely none of the above.

Checklist ✅

• I have done a self-review of my code.
• I have manually tested my code (if applicable).
• I have added/updated any relevant documentation (if applicable).