refactor!: Update .env variables, and add support for conditional output.

[Ta5r]

What

This PR updates environment variable names and descriptions, introduces new variables, updates integration tests accordingly, and modifies documentation to reflect these changes.

Why

These updates are required to support the robust URL PR by ensuring consistency in variable usage, improving clarity in documentation, and refining integration tests.

Related Issue(s):

• For feat: robust handling for internal uri snapwp#111

How

• Updated and added environment variables in the variable registry
• Reordered access functions for better readability and maintainability
• Updated integration tests to validate new and modified variables
• Enhanced documentation to reflect changes in environment variables

Screenshots

Checklist

• I have read the Contribution Guidelines.
• My code is tested to the best of my abilities.
• My code passes all lints (PHPCS, PHPStan, ESLint, etc.).
• My code has detailed inline documentation.
• I have added unit tests to verify the code works as intended.
• I have updated the project documentation as needed.
• I have added a changelog entry for my changes to CHANGELOG.md.

[justlevine]

@Ta5r

I got a bit carried away at 3AM, and instead of just moving the variables around I refactored a lot of internals so:

1. The only vars that get generated are the ones that are needed for the site.
2. The Environment Variables table shows an indicator as to which ones are in the .env file.

PTAL and then check out the Integration test coverage and backfill any tests that may be needed on Generator or VariableRegistry now that it's been refactored (I only converted the tests so they would pass I didn't have time to take a look at the coverage changes.