feat: Explicitly list items to re-export from derive

[Noratrieb]

This allows rustc to give a nice note about activating feature when it has not been activated.

Before:

error[E0433]: failed to resolve: could not find `Parser` in `clap`
 --> ssh/src/main.rs:1:16
  |
1 | #[derive(clap::Parser, Debug)]
  |                ^^^^^^ could not find `Parser` in `clap`

After:

error[E0433]: failed to resolve: could not find `Parser` in `clap`
  --> ssh/src/main.rs:1:16
   |
1  | #[derive(clap::Parser, Debug)]
   |                ^^^^^^ could not find `Parser` in `clap`
   |
note: found an item that was configured out
  --> /home/nora/.cargo/git/checkouts/clap-cf0796c5889852a8/ec804fa/src/lib.rs:93:35
   |
93 | pub use clap_derive::{self, Args, Parser, Subcommand, ValueEnum};
   |                                   ^^^^^^
note: the item is gated behind the `derive` feature
  --> /home/nora/.cargo/git/checkouts/clap-cf0796c5889852a8/ec804fa/src/lib.rs:89:7
   |
89 | #[cfg(feature = "derive")]
   |       ^^^^^^^^^^^^^^^^^^

[epage]

Thanks!