codemirror: enable activeline addon

https://codemirror.net/demo/activeline.html

bt5/erp5_code_mirror/SkinTemplateItem/portal_skins/erp5_code_mirror/code_mirror_support.txt

@@ -17,6 +17,7 @@
 <script type="text/javascript" src="&dtml-portal_url;/codemirror/addon/search/searchcursor.js"></script>
 <script type="text/javascript" src="&dtml-portal_url;/codemirror/addon/search/search.js"></script>
 <script type="text/javascript" src="&dtml-portal_url;/codemirror/addon/search/jump-to-line.js"></script>
+<script type="text/javascript" src="&dtml-portal_url;/codemirror/addon/selection/active-line.js"></script>
 
 <!-- Python autocomplete (Ctrl-Space, see below)
      TODO-arnau: Add ERP5 autocompletion?
@@ -478,6 +479,7 @@
    getTextareaField()[0],
    {mode: mode,
     lineNumbers: true,
+    styleActiveLine: true,
     showTrailingSpace: true,
     tabSize: 2,
     indentWithTabs: false,