feature: Allow overriding with other env files

[e-e]

Instead of managing multiple env files with duplicate data, this adds a way to provide additional env files to use as overrides. The idea here being that there would be a "base" env file, and then environment-specific env files which would override a subset of the base env file values.

There were some auto-formatting changes, but I tried to run them against dartfmt and it said there were no changes to be made 🤷

[java-james]

Thanks for the PR just a note to say aware it's open and will make time to review and see how this aligns with larger .env libraries in regards to usage

[e-e]

Hey @java-james. No pressure on the PR, but just wanted to report back that we are currently successfully using this addition in our app

[SAGV]

This would be so good to have!