Implement multi-layer insert for GraphQL mutations

[konard]

Summary

Implements multi-layer insert functionality for GraphQL mutations, allowing nested object creation as requested in issue #10.

Changes Made

• Fixed LinksObjRelInsert.data field: Changed from recursive LinksObjRelInsert to LinksInsert type to properly support nested object structure
• Enhanced LinksMutation.InsertLink method: Added recursive logic to handle nested from and to objects
• Added .editorconfig file: Created missing configuration file to resolve build issues

Features

Now supports GraphQL mutations with nested object creation:

mutation InsertTriplet {
  insert_links_one(object: { from: { from_id: 1, to_id: 2 }, to_id: 3}) {
    returning {
      id
    }
  }
}

Implementation Details

• When from or to objects are provided with data field, the system first creates those nested links recursively
• The IDs of the created nested links are then used as from_id and to_id for the parent link
• Falls back to direct from_id/to_id values when nested objects are not provided
• Maintains backward compatibility with existing mutation patterns

Test Results

• ✅ Project builds successfully with only expected nullable reference warnings
• ✅ Backward compatibility maintained
• ✅ Recursive nested object creation supported

Fixes #10

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