chore(napi): expose functions which turn raw pointer into External

[zackradisic]

Hi, we recently shipped a native bundler plugin API for Bun, with a Rust crate which exposes a convenient way to create native bundler plugins for Bun (here).

Native bundler plugins in Bun are NAPI modules which expose an additional symbol to a function which implements the bundler plugin code.

To make this nicer to write in Rust, we want to allow users to integrate it with napi-rs.

We want to allow users to use napi-rs External type when writing native bundler plugins. Native bundler plugins can carry around the raw pointer to the napi external.

However, napi-rs does not mark the fields of External as pub or export anyway to go from *mut c_void -> External<T>.

Currently in bun-native-plugin-rs we do something like this to do that:

/// Copy-pasted from napi-rs, not ideal
#[repr(C)]
pub struct TaggedObject<T> {
    type_id: TypeId,
    pub(crate) object: Option<T>,
}

pub unsafe fn external<T: 'static + Sync>(&self) -> PluginResult<Option<&'static T>> {

        // ...
        
        let external: *mut TaggedObject<T> =
            unsafe { (*self.args_raw).external as *mut TaggedObject<T> };
            
         // ...
}

This is obviously bad as napi-rs could change TaggedObject.

So this PR exports function which exposes some logic to go from *mut c_void -> Option<&T>, where T is the inner type of the External<T>.

[Brooooooklyn]

Thank you!

[Brooooooklyn]

@zackradisic released in https://github.com/napi-rs/napi-rs/releases/tag/napi%402.16.14