Support for Rust like #[cfg()] attributes

Maybe instead of adding [configurable URIs](https://github.com/dart-lang/language/blob/main/working/augmentation-libraries/parts_with_imports.md) for parts (in addition to imports and exports), we can add a more flexible feature?

As you can see below, it can be used not only for "part" (modules in Rust), and "imports"; but also can be used to conditionally compile statements. Which might be convenient - declare much platform specific logic in conditional parts, and then conditionally use it.

The main point here is that in Rust this is a universal feature - you can condition anything.
And in Dart currently we have push this feature into every directive separately.

```rust
#[cfg(target_os = "windows")]
mod my_windows;

#[cfg(target_os = "macos")]
mod my_macos;

fn main() {
#[cfg(target_os = "windows")]
my_windows::say_hello_windows();

#[cfg(target_os = "macos")]
my_macos::say_hello_macos();
}
```

`my_macos.rs`
```rust
pub fn say_hello_macos() {
println!("Hello, macOS!");
}
```

`my_windows.rs`
```rust
pub fn say_hello_windows() {
println!("Hello, Windows!");
}
```