Modrinth Collection Support

### Enhancement Type

Improve an existing feature

### Describe the enhancement

Hello!

I was wondering if it would be possible to grab modrinth collections and automatically download mods from them.
This would be good because collections don't have to be manually reviewed by modrinth staff, and they would automatically include all versions of the mods, not just ones that has been added to a modpack. For this reason it would need to use the version env variable to figure out which mods are useable on the current version. I think a minimum_required_mods env variable would be also good for this feature, allowing the user to run the collection in a 'partial' mode, or 'all' mode, in which case it could say that some of the mods are not compatible with the selected mc version.

No files, tests, or entry points are identified. Start by locating the existing modpack or Modrinth download logic and tracing how the version environment variable is handled; the feature is done when collections can be selected, compatible mods are resolved, and partial versus all behavior is defined.

Written by the indexing model from the issue text.