expose the `Plugin` type

Currently, the `Plugin` type is [not exported](https://github.com/egoist/tsup/blob/db7a0225cdd5f782ae65c5abee11a24e1dc55a65/src/index.ts#L33), so I can't do:

```ts
function myPlugin(): Plugin { … }
```

The workaround is to use this type:

```ts
type Plugin = Exclude<import('tsup').Options['plugins'], void>[number]
```