Refactor imports: use file extension, improve tree shaking, update tests

tsconfig.json

@@ -10,6 +10,7 @@
     "esModuleInterop": true,
     "emitDeclarationOnly": true,
     "outDir": "lib",
-    "rootDir": "."
+    "rootDir": ".",
+    "allowImportingTsExtensions": true
   }
 }