Local .env support for Environment Config overrides

Folks often complain about accidentally committing src/Env.elm.

Ideally we should be able to have:

• a valid src/Env.elm with purely test or dummy values
• support for an optional .env where with lamdera live
  • any defined values in .env that match an Env.* config item name, override its value
  • any changes to .env live reload as expected

Considerations for a good design:

• Can we make this delightful, such that you can't accidentally typo an Env.elm config name in .env, and not realise you haven't overriden what you thought you did?

Start by tracing how src/Env.elm values are handled when running lamdera live, then identify where an optional .env could be read and reloaded. Define how matching Env.* names and invalid or unknown names should behave, and verify the design covers dummy defaults, overrides, typo detection, and live reload.