FEATURE: Make legacyExtensions configurable

* Make property public to allow configuration through ruleset.xml
* Provide example configuration in ruleset.xml

Relates: #7

src/Standards/Typo3Update/Sniffs/LegacyClassnames/ClassnameCheckerTrait.php

@@ -37,11 +37,11 @@ trait ClassnameCheckerTrait
      * A list of extension names that might contain legacy class names.
      * Used to check clas names for warnings.
      *
+     * Configure through ruleset.xml.
+     *
      * @var array<string>
      */
-    private $legacyExtensions = [
-        'Extbase',
-    ];
+    public $legacyExtensions = ['Extbase', 'Fluid'];
 
     /**
      * @param string $mappingFile File containing php array for mapping.

src/Standards/Typo3Update/ruleset.xml

@@ -1,4 +1,10 @@
 <?xml version="1.0"?>
 <ruleset name="TYPO3 Update">
     <description>Provides sniffs and fixes for TYPO3 Updates.</description>
+
+    <rule ref="Typo3Update.LegacyClassnames.Instanceof">
+        <properties>
+            <property name="legacyExtensions" type="array" value="Extbase,Fluid,Frontend,Core"/>
+        </properties>
+    </rule>
 </ruleset>