chore: update @actions/core to v3 and convert to ESM

This commit updates the @actions/core dependency from v2 to v3.
The primary change in @actions/core v3 is that it is now an ESM-only package.
To maintain compatibility, the following changes were made:
- Added "type": "module" to package.json to switch the project to ESM.
- Converted index.js from CommonJS to ESM, replacing require with import/export.
- Converted __tests__/index.test.js to ESM to support testing the updated index.js.
- Re-built the production bundle in dist/ using ncc to reflect the changes.
- Updated the main entry point check in index.js to use import.meta.url for ESM compatibility.

This ensures the action continues to function correctly with the latest GitHub Actions toolkit library.

package.json

@@ -2,6 +2,7 @@
   "name": "screeps-deploy-action",
   "version": "0.1.1",
   "description": "Deploys screeps code to the official game or an pirvate server.",
+  "type": "module",
   "main": "index.js",
   "scripts": {
     "start": "node index.js",
@@ -9,7 +10,7 @@
     "build": "ncc build index.js -o dist -m --external utf-8-validate --external bufferutil"
   },
   "dependencies": {
-    "@actions/core": "^2.0.0",
+    "@actions/core": "^3.0.0",
     "glob": "^13.0.0",
     "screeps-api": "^1.7.2"
   },