i18n: Define List item separator as a WP_Locale property.

The list item separator is a locale property, and it doesn't make much sense to translate it separately in multiple projects. This changeset implements the following modifications:

- Define list item separator as a new WP_Locale property
- Add `wp_get_list_item_separator()` as a wrapper for `WP_Locale::get_list_item_separator`
- Replace `$wp_locale->get_list_item_separator()` calls with `wp_get_list_item_separator()`
- Added a compatibility layer for bundled themes

Props SergeyBiryukov, swissspidy, rsiddharth, johnbillion, audrasjb.
Fixes #39733.

Built from https://develop.svn.wordpress.org/trunk@52929


git-svn-id: http://core.svn.wordpress.org/trunk@52518 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/functions.php

@@ -8371,3 +8371,17 @@ function is_php_version_compatible( $required ) {
 function wp_fuzzy_number_match( $expected, $actual, $precision = 1 ) {
 	return abs( (float) $expected - (float) $actual ) <= $precision;
 }
+
+/**
+ * Retrieves the list item separator based on the locale.
+ *
+ * @since 6.0.0
+ *
+ * @global WP_Locale $wp_locale WordPress date and time locale object.
+ *
+ * @return string Locale specific list item separator.
+ */
+function wp_get_list_item_separator() {
+	global $wp_locale;
+	return $wp_locale->get_list_item_separator();
+}