MDL-82444 filter_tidy: advertise required PHP extension.

filter/tidy/environment.xml

@@ -0,0 +1,12 @@
+<?xml version="1.0" encoding="UTF-8" ?>
+<COMPATIBILITY_MATRIX>
+  <PLUGIN name="filter_tidy">
+    <PHP_EXTENSIONS>
+      <PHP_EXTENSION name="tidy" level="optional">
+        <FEEDBACK>
+          <ON_CHECK message="tidyextensionrequired" plugin="filter_tidy" />
+        </FEEDBACK>
+      </PHP_EXTENSION>
+    </PHP_EXTENSIONS>
+  </PLUGIN>
+</COMPATIBILITY_MATRIX>

filter/tidy/lang/en/filter_tidy.php

@@ -25,3 +25,4 @@
 
 $string['filtername'] = 'HTML tidy';
 $string['privacy:metadata'] = 'The HTML tidy plugin does not store any personal data.';
+$string['tidyextensionrequired'] = 'In order to use this filter, the \'tidy\' PHP extension must be installed';