Consider splitting configuration of foreign types into a new `generate!` macro config key

In https://github.com/bytecodealliance/wit-bindgen/pull/972 the default for whether bindings would be generated for foreign (i.e., not in the same package as the target world) types was flipped from being generated by default to not. Users could still opt-into generating code for foreign types by using the `generate` keyword.

The use of a special keyword, however, might not be the most discoverable. Instead we may wish to introduce a new key in the `generate!` macro.

In short the following...

```rust
with: {
"wasi:io/poll": wasi::io::poll,
"some:package/my-interface": generate,
},
```

...would become the following...

```rust
with: {
"wasi:io/poll": wasi::io::poll,
},
generate: [
"some:package/my-interface"
]
```

Commencez par la configuration de la macro generate! décrite dans l’issue et suivez la manière dont la clé with existante gère la valeur generate pour les types étrangers. Comparez les configurations actuelle et proposée, puis vérifiez que la nouvelle clé generate prend en charge le même comportement d’activation opt-in, tandis que with reste utilisé pour les mappings.

Rédigé par le modèle d'indexation à partir du texte de l'issue.