Expose acorn plugin from espree module

The `espree` parser is defined as a plugin for the Acorn parser. This plugin used to be accessible via `espree/lib/espree.js`, but this is now an internal module and cannot be imported directly.

Could [this value](https://github.com/eslint/js/blob/9061c643a7282d0da4d2689482d5b56da83c3dac/packages/espree/espree.js#L61) be exported from the package? Something like `export const acornPlugin = espree;`