tsconfig-ext: add /app, /test, and /stubs/app directories

This is so that they get built and tested, as we'll start running
tests on the ext/ directories from now on.

tsconfig-ext.json

@@ -3,6 +3,9 @@
   "files": [],
   "include": [],
   "references": [
+    { "path": "./app" },
+    { "path": "./stubs/app" },
+    { "path": "./test" },
     { "path": "./ext/app" }
   ],
 }