refactor: Optional development environment separate from production database for testing purposes

[DylanDevelops]

Introduces a development mode activated by the TMPO_DEV environment variable, which uses a separate database directory (~/.tmpo-dev/) to prevent accidental modification of production data. Updates documentation to explain usage, benefits, and database locations for both production and development modes.

Pull Request Checklist

• I have read and followed the contribution guidelines.
• My pull request targets the main branch of tmpo.
• I have tested these changes locally on my machine.

Closes #36

Description

This pull request adds a development mode to the application, allowing developers to safely test changes without affecting real user data. It introduces the TMPO_DEV environment variable, which switches the database location to a separate directory for development purposes. Documentation has been updated to explain how to use this feature and clarify the differences between production and development modes.

Development mode support:

• Added logic in internal/storage/db.go to check the TMPO_DEV environment variable; when set to "1" or "true", the application uses the ~/.tmpo-dev/ directory for its database instead of the default ~/.tmpo/ directory. [1] [2]

Documentation updates:

• Updated CONTRIBUTING.md with a new "Development Mode" section, including usage instructions, database locations, and guidance on making the setting persistent for development.
• Clarified the storage locations for production and development databases in the "Database Storage" section of CONTRIBUTING.md, and added a reference to the new development mode documentation.

[DylanDevelops]

I think this should be helpful; however, I would have liked development environments to use the development directory by default. I may switch this to an "opt-out" system later on.

To make the switch from opt-in to opt-out, I just need to include steps to set TMPO_DEV=0 when building from source for local production purposes.