feat: Add `envconfig` annotation for non-nested structs + `env_prefix` attribute

Hello, I am new to Rust and thought of this feature while using it for another training project. This should allow to parse environment variables that have a common prefix without having to use that prefix in every variables.

I also took the liberty to update the dependencies to their latest version, except for syn because it would require more work, because i feel like it's always better to have the latest updates for any dependencies if it might improve performance or patch some security issues.

I'll gladly take any input if the code, or even the logic, can be improved in any way.