Allow using Rust features to enable WIT features

I would like to be able to do the following:

```rust
wit_bindgen::generate!({
...
features: #[cfg(feature = "a")]{["a"]} #[cfg(not(feature = "a"))]{[]},
});
```

No file, test, or implementation entry point is named. Start by tracing the `wit_bindgen::generate!` macro and its existing WIT feature handling, then define how Rust `cfg(feature = ...)` expressions should select feature lists; done means the requested example works and is covered by tests.

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