[wp-trac] [WordPress Trac] #59137: Add `wp_get_theme_data_custom_templates` function

[WordPress Trac]

#59137: Add `wp_get_theme_data_custom_templates` function
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 Reporter:  oandregal    |      Owner:  oandregal
     Type:  enhancement  |     Status:  assigned
 Priority:  normal       |  Milestone:  6.4
Component:  Themes       |    Version:
 Severity:  normal       |   Keywords:
  Focuses:               |
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 We lack a public function `wp_get_theme_data_custom_templates` to offer
 access to metadata stored under `customTemplates` in `theme.json`.

 ## Why

 There is a no public function to get access to that data, so consumers
 need to resort to private APIs. This follows the steps of
 `wp_get_global_settings`, `wp_theme_has_theme_json`,
 `wp_get_theme_directory_pattern_slugs`, and
 `wp_get_theme_data_template_parts`.

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Ticket URL: <https://core.trac.wordpress.org/ticket/59137>
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