Add support for the Horizon server wrapper in Plugins library

[ProgMEM-CC]

Please confirm the following.

• I checked the existing issues for duplicate feature requests

What parts of Modrinth is your feature request related too?

Modrinth.com website

Is your suggested feature related to a problem? Please describe.

Nil, not related to a problem.

Describe the solution you'd like

Add "Horizon" as a seperate "Loader" in the plugins section so we can upload a plugin meant for horizon.

Horizon info: https://docs.canvasmc.io/horizon/introduction/

Describe alternatives you've considered

nil

Additional context

nil