Add `wp_get_theme_data_custom_templates` function

[oandregal]

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.

Part of ​https://github.com/WordPress/gutenberg/issues/45171
Trac ticket: https://core.trac.wordpress.org/ticket/59137

## What

This PR adds a new 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.

## How

It creates a new function that uses the private calls and substitutes all the instances of the old one in the codebase.

## How to test

TBD.

## Commit message

Themes: add `wp_get_theme_data_custom_templates` function.

Adds a new public function, `wp_get_theme_data_custom_templates` that returns the `customTemplates` defined by the active theme from `theme.json`. It also substitutes the usage of private APIs by this new API.

Fixes #59137

[audrasjb]

Thanks for the ticket and patch,
Marking this as ready for commit.

This function is missing a return statement and its @return tag is incorrect. Can you check and correct those please?

Thanks, @johnbillion for catching the issue. Both things addressed.

Ah correct.
The change looks good to me, but we usually add a line break on long docblock sentences to avoid having too much characters on the same line

The change looks good to me, but we usually add a line break on long docblock sentences to avoid having too much characters on the same line

Added the line break as well.

Looking for how we do @return in other functions, I've noticed this ​https://github.com/WordPress/wordpress-develop/pull/5033

@johnbillion when you have the time, can you clear up the blocker so I can commit this?

[oandregal]

In 56413:

Themes: add wp_get_theme_data_custom_templates function.

Adds a new public function, wp_get_theme_data_custom_templates that returns the customTemplates defined by the active theme from theme.json. It also substitutes the usage of private APIs by this new
API.

Props johnbillion, audrasjb.
Fixes #59137

Committed at https://core.trac.wordpress.org/changeset/56413.