Editor: Conditionally enable the new default rendering mode for Pages

[Mamaduka]

What?

Part of #68684.

PR updates logic to enable a new default mode for pages and only enables it for block-based themes.

Why?

• The users get the most out of this rendering mode when using block themes.
• It will eventually work for hybrid themes, but there are some known issues that I'm not sure we can resolve for WP 6.8 (but I'll try my best). See Wrong template displayed for hybrid themes in the post template editor #64341.
• This is also the original direction, which got lost during refactorings. See Feature: Set editor rendering mode by post type #62304.

See: WordPress/wordpress-develop#8123 (comment)

Testing Instructions

Using a block theme.

1. Open a page.
2. Confirm that it loads with a template-locked view.
3. Doesn't change the current behavior.

Using a hybrid theme.

1. Open a page.
2. Create a block-based template for it and save it.
3. Confirm that it doesn't load with a template-locked view

Testing Instructions for Keyboard

Same.

Screenshots or screencast

[github-actions]

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Co-authored-by: Mamaduka <mamaduka@git.wordpress.org>
Co-authored-by: fabiankaegy <fabiankaegy@git.wordpress.org>
Co-authored-by: audrasjb <audrasjb@git.wordpress.org>
Co-authored-by: swissspidy <swissspidy@git.wordpress.org>

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[fabiankaegy]

I think this is the right compromise for 6.8 and we should ship this in the Betas with a Call for testing 👍 :)

[audrasjb]

The code logic here looks good to me 👍

[Mamaduka]

Thanks everyone for the feedback 🙇