Make the project's parent path configurable when it supports multiple platforms

[xpple]

Please confirm the following.

• I checked the existing issues for duplicate feature requests
• I have checked that this feature request is not on our roadmap

What parts of Modrinth is your feature request related too?

Website

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

Currently, when a project supports multiple platforms like Fabric and Paper, the parent path of the project will default to /plugin. For example, my mod supports both these platforms, and its URL is fixed to https://modrinth.com/plugin/betterconfig/. In general this choice makes sense, when a project supports both platforms it can only do as much as a plugin can. Despite this, it would be nice to be able to make this configurable. Note that using /mod in the URL is possible; it will redirect to /plugin.

Describe the solution you'd like

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Describe alternatives you've considered

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Additional context

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