chore: replace dotenvy with dotenvor

[JohnTitor]

dotenvy's last release was made almost 3 years ago, and the last commit is over 1 year ago.
As I'm not sure if it'll be revived in the future, I've created a small dotenv crate for examples.
unsafe required because of edition 2024.

[robjtede]

this being a repo which people often copy from blindly, i don't agree with us adding a dependency which requires unsafe in so many projects

[JohnTitor]

@robjtede
Note that it was already unsafe technically even with dotenvy, it just doesn't expose unsafety. See: https://doc.rust-lang.org/edition-guide/rust-2024/newly-unsafe-functions.html#stdenvset_var-remove_var
But I'm agree with your concern and I'll explore a similar solution in allan2/dotenvy#112.

[JohnTitor]

@robjtede I've been thinking this for a while but it's a bit difficult to make a choise and I'd like to hear your thoughts.
If we prefer in-memory loading way, it doesn't work well with other libs like DB drivers which need env-loading strictly.
If we prefer proc-macros (or a way like dotenvy), it just hides unsafety, but user still needs to ensure on env processing.
I think it's the best way just to use unsafe methods here, it aligns why Rust makes it unsafe.