feat: Index fields

[demshy]

Summary

As described in issue #7381 we need a way to have a specific field set for index files in collections. More info coming as we smooth out the details and clean up the code.

Test plan

Add the following config to the collection options. Pattern is required. Fields are optional, setting them will result in a different entry form for this file. Label is also optional and will add prefix to the title in the entry list.

    index_file: {
      pattern: "^index$",
      fields: [
        ...
      ],
      label: 'Index'
    }

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