wpcom-block-editor: Support getting the canvas mode from the query string after GB 19.6

[arthur791004]

Related to #95960

Proposed Changes

• Support getting the canvas mode from the query string because the getCanvasMode selector is removed after GB 19.6

Why are these changes being made?

• The getCanvasMode selector is removed after GB 19.6 and causes the error.

Testing Instructions

• Sandbox widgets.wp.com
• Apply changes to your site by either one of below
  • Run yarn dev --sync
  • Run install-plugin.sh wpcom-block-editor fix/wpcom-block-editor-canvas-mode on your sandbox
• Go to Theme Showcase on your free site
• Pick any paid block theme
• Preview the theme in the editor
• Make sure to disable the cache of https://widgets.wp.com/wpcom-block-editor/wpcom.editor.min.js
• Make sure you can see the upgrade notice of the live preview on both view and edit mode

Pre-merge Checklist

• Has the general commit checklist been followed? (PCYsg-hS-p2)
• Have you written new tests for your changes?
• Have you tested the feature in Simple (P9HQHe-k8-p2), Atomic (P9HQHe-jW-p2), and self-hosted Jetpack sites (PCYsg-g6b-p2)?
• Have you checked for TypeScript, React or other console errors?
• Have you used memoizing on expensive computations? More info in Memoizing with create-selector and Using memoizing selectors and Our Approach to Data
• Have we added the "[Status] String Freeze" label as soon as any new strings were ready for translation (p4TIVU-5Jq-p2)?
• For changes affecting Jetpack: Have we added the "[Status] Needs Privacy Updates" label if this pull request changes what data or activity we track or use (p4TIVU-aUh-p2)?

[github-actions]

Calypso Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-123467

Jetpack Cloud live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-123467&env=jetpack

Automattic for Agencies live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-123467&env=a8c-for-agencies

[matticbot]

This PR does not affect the size of JS and CSS bundles shipped to the user's browser.

Generated by performance advisor bot at iscalypsofastyet.com.

[matticbot]

This PR modifies the release build for the following Calypso Apps:

For info about this notification, see here: PCYsg-OT6-p2

• wpcom-block-editor

To test WordPress.com changes, run install-plugin.sh $pluginSlug fix/wpcom-block-editor-canvas-mode on your sandbox.

[fushar]

It's working great, thanks for the patch!