add assertIsSetup() and use it from OSThread constructor

fixes nasty bug @mc-hamster discovered with plugin order of operations

src/plugins/Plugins.h

@@ -0,0 +1,6 @@
+#pragma once
+
+/**
+ * Create plugin instances here.  If you are adding a new plugin, you must 'new' it here (or somewhere else)
+ */
+void setupPlugins();