start to use more unified way of scripts organization

extensions.gradle

@@ -0,0 +1,8 @@
+allprojects {
+    ext {
+        mppProjectWithSerializationPresetPath = "${rootProject.projectDir.absolutePath}/mppProjectWithSerialization.gradle"
+        mppJsProjectPresetPath = "${rootProject.projectDir.absolutePath}/mppJsProject.gradle"
+
+        publishGradlePath = "${rootProject.projectDir.absolutePath}/publish.gradle"
+    }
+}