feat(canvas): expose block/section/content jump to extensions

[mhaack]

Summary

• Extensions (BYO-plugin iframes) can now trigger the same "jump to a block/content item/section" behavior that clicking a row in the ew-page-outline panel already does.
• Adds a scrollTo action to the existing extension MessageChannel protocol (blocks/canvas/ew-panel-extensions/iframe-protocol.js), relaying onto the existing canvasBus.editorSelectState/editorProseSelectState channels — no new event mechanism.
• A section has no selectable identity of its own, so it resolves to its first block/content item via a new resolveSectionTarget helper in editor-utils.js.
• Generalizes ew-editor-doc.js's WYSIWYG-sync broadcast from "outline-sourced only" to "any non-doc source", so extension-triggered jumps also sync the WYSIWYG overlay.
• Pairs with feat(sdk): add actions.scrollTo(target) for extensions da-nx#700 (adds the actions.scrollTo(target) SDK entry point extensions call) — both sides are additive and inert alone; neither breaks anything shipped independently.

Test:

• https://selsec--da-live--adobe.aem.live/canvas?nx=selsec#/exp-workspace/frescopa/coffee

Test plan

• Unit tests added for resolveSectionTarget (block target, content target, empty section, out-of-range, cache-clear)
• Unit tests added for the scrollTo handler in iframe-protocol.js (block, content, section-resolved-to-block, out-of-range section, unrecognized type)
• Unit tests added for ew-editor-doc.js's _onEditorSelectState (doc/outline/extension sources)
• Full unit suite green: 2017 passed, 0 failed

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Commits

• e32f0a9 ✅ (latest)
• 814ebac ✅
• ed77331 ✅

[sharanyavinod]

Since this could be coming from an extension, maybe we could validate the blockindex prior to doing the broadcast?