Add support custom template for showing remaining length

+++ b/src/Plugin/Field/FieldWidget/TextFieldCounterWidgetTrait.php
@@ -59,7 +71,16 @@ trait TextFieldCounterWidgetTrait {
+  public function addFieldFormElement(&$element, $entity, $fieldDefinition, $delta, $maxlength, $position, $template) {
+    $template = str_replace(
+      [
+        '@length',
+        '@remaining_count',
+      ],
+      [
+        '<span class="length">@length</span>',
+        '<span class="remaining_count">@remaining_count</span>',
+      ], $template);

These replace will override the template which will be used in the Drupal.t() later, that may create confusion with translations. Because a user will want to translate his template without any HTML code.
Maybe will be better to add this tags directly in Drupal.l() when the placeholders set, if they really need here. What do you think?

I went at this a different way. In the interests of keeping all settings in a single location, I added this as a setting for the widget, so it can be configured on a per-instance basis. Available message variables are below. Note that the remaining count and current count must be wrapped in an element with the HTML class in the brackets, in order for the value to be auto updated as the user types.

@remaining_count (.remaining_count)
@current_count (.current_count)
@maxlength