Normalize UTF-8 charset slug detection.

There are several exist places in Core that attempt to detect if a blog charset
is UTF-8. Each place attempts to perform the same check, except the logic is
spread throughout and there's no single method provided to make this
determination in a consistent way. The `_canonical_charset()` method exists,
but is marked private for use.

In this patch the new `unicode` module provides `is_utf8_charset()` as a method
taking an optional charset slug and indicating if it represents UTF-8,
examining all of the allowable variants of that slug. Associated code is
updated to use this new function, including `_canonical_charset()`. If no slug
is provided, it will look up the current `get_option( 'blog_charset' )`.

Finally, the test functions governing `_canonical_charset()` have been
rewritten as a single test with a data provider instead of as separate test
functions.

Developed in https://github.com/WordPress/wordpress-develop/pull/6535
Discussed in https://core.trac.wordpress.org/ticket/61182

Fixes #61182.
Props dmsnell, jonsurrell.

Built from https://develop.svn.wordpress.org/trunk@58147


git-svn-id: http://core.svn.wordpress.org/trunk@57612 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/options.php

@@ -160,7 +160,7 @@ $allowed_options['privacy'] = array();
 
 $mail_options = array( 'mailserver_url', 'mailserver_port', 'mailserver_login', 'mailserver_pass' );
 
-if ( ! in_array( get_option( 'blog_charset' ), array( 'utf8', 'utf-8', 'UTF8', 'UTF-8' ), true ) ) {
+if ( ! is_utf8_charset() ) {
 	$allowed_options['reading'][] = 'blog_charset';
 }