Feat: Support .env files in nested project directories

[grahamplata]

This PR updates the Project parser to handle multiple .env files. Nested .env files will be parsed and merged with variables loaded from other .env files.

Issue: #6490

Tested with devtool

• Start local and cloud
• Create .env in project root
• Deploy and Validate
• Create .env in another project dir models/
• Deploy and validate

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated
• Intend to cherry-pick into the release branch
• I'm proud of this work!