theme.json: Add support for description in customTemplates

[dashkevych]

Default templates, such as Single, Page, Index, ect, come with a description text, while custom templates do not have such text.

It would be great if we were able to display a description text in custom templates, set via theme.json, to provide more information about the custom template.

For instance:

"customTemplates": [
    {
        "name": "blank",
        "postTypes": [
            "page",
            "post"
        ],
        "title": "Blank",
        "description": "Empty template."
    },
}

[github-actions[bot]]

Help us move this issue forward. This issue is being marked stale since it has no activity after 15 days of requesting more information. Please add info requested so we can help move the issue forward. Note: The triage policy is to close stale issues that need more info and no response after 2 weeks.

[t-hamano]

@dashkevych Thanks for the suggestion. I think this is the same as #44097, so let's close this issue and focus the discussion on #44097.