add jest as extension devDep, go back to using makefile foreach syntax

Signed-off-by: Sebastian Malton <sebastian@malton.name>

extensions/example-extension/package.json

@@ -18,6 +18,7 @@
   },
   "devDependencies": {
     "@k8slens/extensions": "file:../../src/extensions/npm/extensions",
+    "jest": "^26.6.3",
     "ts-loader": "^8.0.4",
     "typescript": "^4.0.3",
     "webpack": "^4.44.2"