Editor: Add an extra check before enabling the new default rendering mode for Pages

[Mamaduka]

What?

This is a small follow-up to #69160.

PR hardens condition before enabling template-locked as default rendering mode for Pages.

Why?

While it should be an edge case for a block theme not to support block-templates, it's still technically doable. See: #67875 (comment).

Testing Instructions

1. Open a page.
2. Confirm that it loads with a template-locked view.
3. Doesn't change the current behavior.
4. Remove block-templates theme support.
5. Reload the page.
6. Confirm that it loads in post-only mode.

Testing Instructions for Keyboard

Same.

[github-actions]

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

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[fabiankaegy]

Looks good to me 👍

though really I would expect that then support to be a check that happens inside the is_block_theme function because what is a blink theme if not block templates?

[Mamaduka]

@fabiankaegy, I think the current behavior is for legacy reasons. WP shipped block-templates before Site Editor and block themes, which started using the existing system by enabling block templates (see wp_enable_block_templates).

The wp_is_block_theme can also be called before any theme support flags are setup since it only checks the existence of files in themes.