Add missing default template areas to the Page/Post Editor settings

[Mamaduka]

The default template area settings shown when editing a template in the "Post/Page" editor got lost during backporting for 5.9.

In the Gutenberg plugin, we're filtering block_editor_settings_all to add this setting for block themes - ​https://github.com/WordPress/gutenberg/blob/trunk/lib/editor-settings.php#L20-L32.

Testing

Running following snippet in DevTools console currently returns an empty array:

wp.data.select('core/editor').__experimentalGetDefaultTemplatePartAreas();

After the fix, it should return the following values:

[
    {
        "area": "uncategorized",
        "label": "General",
        "description": "General templates often perform a specific role like displaying post content, and are not tied to any particular area.",
        "icon": {},
        "area_tag": "div"
    },
    {
        "area": "header",
        "label": "Header",
        "description": "The Header template defines a page area that typically contains a title, logo, and main navigation.",
        "icon": {},
        "area_tag": "header"
    },
    {
        "area": "footer",
        "label": "Footer",
        "description": "The Footer template defines a page area that typically contains site credits, social links, or any other combination of blocks.",
        "icon": {},
        "area_tag": "footer"
    }
]

Testing instructions can be found in the Trac ticket.

I've not added this setting to the get_default_block_editor_settings since Template Parts are only supported in the Template Editing Mode.

Gutenberg issue: ​https://github.com/WordPress/gutenberg/issues/37443
Trac ticket: https://core.trac.wordpress.org/ticket/54679

/cc @gziolo, @hellofromtonya

[costdev]

Test Report

Env

• WordPress 5.9 Beta 4
• Chrome 96.0.4664.110
• Windows 10 (Local)
• Theme: Twenty Twenty-Two
• Plugin: None activated

Steps to test

1. Enable a block theme, such as Twenty Twenty-Two.
2. Navigate to Pages > Add New.
3. To the right, expand the Template section.
4. Click Edit.
5. Insert a Template Part.
6. Click Choose existing. In the AREA: headings, you should see UNDEFINED.
7. Apply ​PR 2076.
8. Repeat steps 2-6. The AREA: headings should now show the correct name.

Results

• Before ​PR 2076: UNDEFINED is shown.
• After ​PR 2076: The correct name is shown.
• PR works as expected 👍

[SergeyBiryukov]

In 52409:

Editor: Add missing default template areas to the post/page editor settings.

This ensures that default template area settings are displayed when editing a template in the post/page editor, which previously got lost during backporting for 5.9.

Follow-up to [52232].

Props Mamaduka, costdev, gziolo.
Fixes #54679.

Thanks for the PR! Merged in https://core.trac.wordpress.org/changeset/52409.