Fix npm publish

octokit/webhooks.js#552 broke `npm publish` in
this repository by introduces `.ts`extensions to some imports (which
caused `npm publish` to fail with "error TS2691: An import path cannot
end with a '.ts' extension"). This fixes that.

tsconfig.json

@@ -8,6 +8,9 @@
     "strict": true,
     "target": "es2019"
   },
+  "ts-node": {
+    "esm": true
+  },
   "include": [
     "src/**/*"
   ]