Pushing column mapping definition to widget's manifest

[berhalak]

Column mappings live in the widget code, inside grist.ready(). Grist only
sees them after the widget loads. An assistant that adds a widget to a page
cannot see them at all, so it cannot map the columns.

This PR adds the mappings to package.json, so they end up in
manifest.json. This is the new place to define columns for a widget.

The old mappings in grist.ready() stay, so nothing breaks. New widgets can
use the manifest only.

Not related: rendervideo used label instead of title, now fixed.

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[paulfitz]

Thanks @berhalak, looks pretty good.

In passing, could you fix the existing issues of a double grist.ready in flashcards:

  grist.ready({
    columns: [
      { name: "Question", type: 'Text', title: "Question Column"},
      { name: "Answer", type: 'Text', title: "Answer Column"},
    ],
    requiredAccess: 'read table'
  });
  grist.ready();

and in maps/page.js there's a different duplication:

    { name: "Address", type: 'Text', optional, optional},

[paulfitz]

thanks @berhalak !