packages/cli: separate handlers for different template files in diff

packages/cli/package.json

@@ -29,6 +29,7 @@
     "backstage-cli": "bin/backstage-cli"
   },
   "devDependencies": {
+    "@types/diff": "^4.0.2",
     "@types/fs-extra": "^8.1.0",
     "@types/html-webpack-plugin": "^3.2.2",
     "@types/inquirer": "^6.5.0",
@@ -58,6 +59,7 @@
     "chokidar": "^3.3.1",
     "commander": "^4.1.1",
     "dashify": "^2.0.0",
+    "diff": "^4.0.2",
     "eslint-plugin-import": "^2.20.2",
     "eslint-plugin-monorepo": "^0.2.1",
     "fork-ts-checker-webpack-plugin": "^4.0.5",