The Pino transport could not be used as a transport (that is, as
initialised by `pino.transport()`, but only as a destination, and
only in the main thread.

This commit makes the Pino transport communicate with the extension
directly, which should work from any worker thread -- the assumption
being that the extension has been started by the main thread.

This works around the fact that the client cannot be serialised and
sent across threads.

This allows for the Pino transport to be used like other transports,
and _alongside_ other transports:

```js
const logger = pino({
  transport: {
    targets: [
      { target: "@appsignal/nodejs/pino", options: { group: "pino" } },
      { target: ... }
    ]
  }
})
```

The `pino/index.js` file has been added as a convenience to allow for
the target to be referred to as `@appsignal/nodejs/pino`, which is
nicer-looking than the transport's real location inside `dist/`.