fix(richtext): preserve block images in list items

[marthesselink]

Summary

Fixes #7895.

Configure Plate's classic list plugin to accept shortcode children. Decap represents block editor components, including the built-in image component, as shortcode nodes. Without this configuration, Plate's list normalizer removes an image nested in a list item when the visual editor initializes, and the next save persists that data loss.

This change:

• preserves block images nested in list items;
• adds a regression test for the Markdown parse, normalize, and serialize round trip;
• maps Plate's React entry points in Jest and allows the nested ESM dependencies needed by that test.

Reproduction

Opening and saving this Markdown in the rich-text editor:

1. First step.
2. Last step.

   ![Screenshot](/img/screenshot.png)

previously produced:

1. First step.
2.

The regression test fails with that exact output before the fix and preserves the original Markdown after the fix.

Test plan

• npx jest packages/decap-cms-widget-richtext/src/RichtextControl/VisualEditor.spec.js --runInBand
• npx jest packages/decap-cms-widget-richtext --runInBand
• npm run format
• npm run test
• npm run build

Checklist

• I have read the contribution guidelines.
• I have added a regression test for the bug fix.
• I have linked the issue fixed by this pull request.

[yanthomasdev]

LGTM! Thanks!