Specify attributes using comments

The behavior can often be customized via attributes as well. For example, to make a function synchronous on Dart side, we can put #[frb(sync)] on that function.

Not only can we write down #[frb(something)], but we can also write comments /// flutter_rust_bridge:something to do the same thing.

The latter is especially useful when the former cannot be used, such as when the target is a mod, or when the crate does not have dependency on flutter_rust_bridge.

Most of the time, the latter is equivalent to the former; but for things like #[frb(external)], which has to act as a macro to generate some code, the latter cannot be used. But this can be easily spotted since it will have compile-time messages.

Example

/// flutter_rust_bridge:ignore
pub fn f() {}