Switch build output to ESM format and update entrypoints

Changed the build script to output ES modules instead of CommonJS. Updated action.yml to reference .js files instead of .cjs, and clarified proxy support logic in bootstrap.js.

package.json

@@ -8,7 +8,7 @@
     "node": ">=24.4.0"
   },
   "scripts": {
-  "build": "esbuild bootstrap.js main.js post.js --bundle --outdir=dist --out-extension:.js=.cjs --platform=node --packages=bundle",
+  "build": "esbuild bootstrap.js main.js post.js --bundle --outdir=dist --format=esm --platform=node --packages=bundle",
     "test": "c8 --100 ava tests/index.js",
     "coverage": "c8 report --reporter html",
     "postcoverage": "open-cli coverage/index.html"