feat: Allow block discriminator field in view columns

[antoniocosta]

Summary

This PR enables displaying the block discriminator field (e.g., _block) in collection view columns.

Problem

When using block fields, there's no way to display the block type in the list view. Users who want to show which block type each entry uses (e.g., "image", "video", "gallery") currently cannot do so because:

1. getFieldByPath only traverses into object type fields, not block types
2. Block fields are explicitly excluded from view columns

Solution

Modified getFieldByPath in lib/schema.ts to recognize the block discriminator field (defaulting to _block, or a custom blockKey if configured). When a path like content._block is requested, it returns a virtual string field definition.

Usage

In .pages.yml:

fields:
  - name: content
    type: block
    blocks:
      - name: image
        # ...
      - name: video
        # ...

view:
  fields: [title, date, content._block]

This will display "image", "video", etc. in the list view column.

Changes

• lib/schema.ts: Extended getFieldByPath to handle block discriminator fields (~10 lines added)

Testing

Tested with a collection of 3300+ posts using various block types (image, video, gallery, quote, link, post). The column correctly displays the block type for each entry.