feat(core): one-to-many references

[plushdohn]

The backend for this should be fairly straightforward (the $lookup aggregation operator already returns an array), the frontend is more involved as we need to essentially manage arrays in the forms for the first time.

The API would look something like:

fields: {
  categories: {
    type: "reference",
    collection: "categories",
    many: true
  }
}

The type should then be inferred just as it currently is for reference fields, but as an array. The include property should need no changes.

An appropriate example for this feature would be adding a comment section to our blog example, which would also highlight server actions and collection-level access control.