refactor: extract FlutterExtension into own file

src/main/groovy/com/fahrecker/gradle/FlutterExtension.groovy

@@ -0,0 +1,25 @@
+package com.fahrecker.gradle
+
+class FlutterExtension {
+    final Property<String> flutterVersion
+    final Property<String> flutterChannel
+    final Property<String> flutterSdkPath
+
+    FlutterExtension(Project project) {
+        flutterVersion = project.objects.property(String)
+        flutterChannel = project.objects.property(String)
+        flutterSdkPath = project.objects.property(String)
+    }
+
+    void setFlutterVersion(String version) {
+        flutterVersion.set(version)
+    }
+
+    void setFlutterChannel(String channel) {
+        flutterChannel.set(channel)
+    }
+
+    void setFlutterSdkPath(String path) {
+        flutterSdkPath.set(path)
+    }
+}

src/main/groovy/com/fahrecker/gradle/FlutterGradlePlugin.groovy

@@ -40,28 +40,4 @@ class FlutterGradlePlugin implements Plugin<Project> {
             return ['flutter'] + args
         }
     }
-}
-
-class FlutterExtension {
-    final Property<String> flutterVersion
-    final Property<String> flutterChannel
-    final Property<String> flutterSdkPath
-
-    FlutterExtension(Project project) {
-        flutterVersion = project.objects.property(String)
-        flutterChannel = project.objects.property(String)
-        flutterSdkPath = project.objects.property(String)
-    }
-
-    void setFlutterVersion(String version) {
-        flutterVersion.set(version)
-    }
-
-    void setFlutterChannel(String channel) {
-        flutterChannel.set(channel)
-    }
-
-    void setFlutterSdkPath(String path) {
-        flutterSdkPath.set(path)
-    }
 }