Bootstrap/Load: Introduce is_*_admin_screen() aliases for is_*_admin() function family.

Following the introduction of `is_login_screen()` in [53884], it is time to reconsider the `is_*_admin()` functions:

* `is_admin()`: Determines whether the current request is for an administrative interface page.
* `is_blog_admin()`: Whether the current request is for a site's administrative interface, e.g. `/wp-admin/`.
* `is_network_admin()`: Whether the current request is for the network administrative interface, e.g. `/wp-admin/network/`.
* `is_user_admin()`: Whether the current request is for a user admin screen, e.g. `/wp-admin/user/`.

For someone new to WordPress, these names can be quite confusing, especially the last one. When using these functions, one always needs to remember that they don't actually check if the current user is a site administrator.

To complicate things further, there is one more similarly named function that does exactly the latter:
* `is_super_admin()`: Determines whether user is a site admin.

With the above in mind, this commit introduces aliases that better match the functionality and allow for more descriptive code:

* `is_admin()` → `is_admin_screen()`
* `is_blog_admin()` → `is_site_admin_screen()`
* `is_network_admin()` → `is_network_admin_screen()`
* `is_user_admin()` → `is_user_admin_screen()`

Additionally, `is_super_admin_user()` is introduced as an alias for `is_super_admin()`:
* `is_super_admin()` → `is_super_admin_user()`

Plugins and themes are encouraged to start using the newer function names to make code self-descriptive and bring more clarity. The older names are not deprecated at this time, though it may be up for discussion in the future.

Follow-up to [2481], [6412], [12393], [12732], [15558], [15481], [15746], [53884].

Props jrf, tobifjellner, SergeyBiryukov.
See #56400.
Built from https://develop.svn.wordpress.org/trunk@54259


git-svn-id: http://core.svn.wordpress.org/trunk@53818 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/capabilities.php

@@ -1084,6 +1084,22 @@ function get_super_admins() {
 	}
 }
 
+/**
+ * Determines whether user is a site admin.
+ *
+ * @since 6.1.0
+ *
+ * This function is an alias for is_super_admin().
+ *
+ * @see is_super_admin()
+ *
+ * @param int|false $user_id Optional. The ID of a user. Defaults to false, to check the current user.
+ * @return bool Whether the user is a site admin.
+ */
+function is_super_admin_user( $user_id = false ) {
+	return is_super_admin( $user_id );
+}
+
 /**
  * Determines whether user is a site admin.
  *