fix: expose discovery as factory

[achingbrain]

To use the same interface as other libp2p components, expose the discovery module as a factory function.

[github-actions[bot]]

:tada: This PR is included in version @libp2p/webrtc-star-v5.0.1 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot :package::rocket: