Fix the classic block TinyMCE init array

[azaozz]

The TinyMCE init array cannot be JSON encoded safely like in $scripts->localize(). Some setting values can be (js) functions, other may be passed as objects or arrays in the tiny_mce_before_init filter. It has to be converted from php to js "by hand" and outputted as raw js from php.

[azaozz]

To test:

add_filter( 'tiny_mce_before_init', 'my_tiny_mce_before_init' );
function my_tiny_mce_before_init( $init ) {
	$init['init_instance_callback'] = 'function(editor) { console.log("Editor: " + editor.id + " is now initialized.") }';
	return $init;
}

[azaozz]

Also opened https://core.trac.wordpress.org/ticket/45221 for core.

This is a regression and a blocker for WP 5.0.