Add pest_derive optional dependency

[kvark]

Would be good to avoid depending on both pest and pest_derive, just for convenience. Just like Serde, pest with pest_derive feature could re-export the macros.

[hellow554]

This is something I wanted since I started using pest.
The problem is, that everything (derive, generator and meta) depends on pest. That's a problem, because if you start including pest_derive in pest you'll get a cyclic depency.

It would be nice if somebody could restructure the crates so they don't depend on pest anymore.

[syddo]

i'm seeing an error "unresolved extern crate, can't find crate for pest_derive can't find crate"
in pest version 2.1.3 using the example in its documentation.

am i missing something?

[dunkyl]

@syddo super late, but, I got the same issue. It was fixed by specifying the Rust edition in Cargo.toml (edition = "2021" under [package])