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.

dist/package.json

@@ -0,0 +1,3 @@
+{
+  "type": "module"
+}