editor: Fixing bug that caused infinite loop during preview

[berhalak]

When an inner widget was reporting some mandatory columns to map, the builder was causing infinite loop. More in comment, but here is summary.

• Builder was sending ready message to Grist, without any mapped columns
• Grist was rendering the widget, sending JS/HTML back
• Builder was creating inner widget, which called ready method and reported some mandatory columns to map
• Grist unloads builder, as columns where not mapped
• User mapped a column
• Grist loads builder
• Builder reports back to Grist that no columns to map (and no mandatory columns anymore).
• Grist unloads builder once again (as mandatory option has changed, and Grist is sensitive here).
• Grist sees no mandatory columns, so shows builder once again.
• Builder, reports back no mandatory columns, creates inner widget, this inner widget reports mandatory columns
• And it loops

Here the basic idea was not to call the configure endpoint from the builder, instead builder is injecting its configuration in the configure call from the inner widget, that way Grist sees only one configure call, and columns mapping stays consistent during reloads.

Not related:

• Some dead code is removed
• api_deps.js is loaded with monaco
• moncaco scripts are loaded in parallel

Here is a document with this builder and markdown widget's content inside (with mandatory column):
https://public.getgrist.com/d1t1x4BvxuMK/JsFiddle

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