Interpolate bash-style variable expansion in .env files

[publicmatt]

It would be nice if the behavior when loading dotenv files matched what's possible with the python-dotenv package:

The syntax of .env files supported by python-dotenv is similar to that of Bash:

# Development settings
DOMAIN=example.org
ADMIN_EMAIL=admin@${DOMAIN}
ROOT_URL=${DOMAIN}/app

[tr11]

Maybe this could be added if we specify a different interpolation type. Right now we use the str.format specifications: https://github.com/tr11/python-configuration?tab=readme-ov-file#string-interpolation