feat: remove Full Site Editor from menu bar

[brunodeangelis]

This PR adds an option to disable the menu item Appearance -> Editor (beta)

There is a plugin that does this better, but the code is not as straightforward as I would've liked to port it here, so this is only what I managed to do.

This code has no redirection from wp-admin/site-editor.php to admin_url() because I couldn't get the usual snippet to work:

add_action( 'template_redirect', function () {
	global $pagenow;

	if ( $pagenow === 'site-editor.php' ) {
		wp_redirect( admin_url() );
		exit;
	}
} );

Any ideas are welcomed :)