add reason to entitydismountevent

Is your feature request related to a problem?

I want to prevent dismounting when entering water

Describe the solution you'd like.

A entitydismountevent reason, eg Reason.IN_WATER

Describe alternatives you've considered.

Forking paper

Agreements

• I have searched for and ensured there isn't already an open issue regarding this.
• I have ensured the feature I'm requesting isn't already in the latest supported Paper build.

Other

No response

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.