CSS classes configuration form field

#2 works for me. Moving to RTBC

A couple of nitpicks with patch #2:

1. +++ b/config/schema/manage_display.schema.yml
   @@ -4,6 +4,9 @@ field.formatter.settings.title:
   +    tag_css_classes:
   

   I don't think this new setting name needs the tag_ prefix.

2. +++ b/src/Plugin/Field/FieldFormatter/TitleFormatter.php
   @@ -70,7 +79,8 @@ class TitleFormatter extends StringFormatter {
   +      $item['#prefix'] = ($tag_css_classes !== '') ? "<$tag class='$tag_css_classes'>" : "<$tag>";
   

   Would probably be better to use !empty() here.

Updated patch that addresses items in #6.

Created a new MR.

Added changes as a mr and updated the settingsSummary() method in order not to show the summary for classes if the field is empty.

Thanks for the contributions. For sure I can see that some sites would wish for this feature I even have a feeling that someone requested it before although I can't find the issue now.

This module aims to get the formatters added to Drupal Core. When developing the formatters I aimed to copy the Core formatters as closely as possible. Core mostly doesn't add a lot of extra configuration options, so I expect they wouldn't accept the option in this feature request. Hence it seems like we shouldn't add it here. What does anyone else think?

Perhaps can use this https://www.drupal.org/project/field_formatter_class → ?