chore: update @actions/core to v3 and migrate to ESM (#73)

This PR updates the `@actions/core` dependency from v2 to v3.0.0.

### Major Changes:
- **Dependency Update**: Updated `@actions/core` to `v3.0.0` in `package.json`.
- **ESM Migration**: Converted the entire project to ECMAScript Modules (ESM). This was necessary because `@actions/core` v3 is an ESM-only package and does not provide CommonJS exports.
- **`package.json` updates**: Added `"type": "module"` and updated the dependency version.
- **Code Refactor**:
    -   Converted `index.js` to use `import` and `export`.
    -   Updated the main execution check to use `import.meta.url` for ESM compatibility.
    -   Converted `__tests__/index.test.js` to use ESM imports for compatibility with the updated source code.
- **Production Build**: Re-generated the `dist/index.js` bundle using `ncc` to reflect the changes.
- **Node.js Version**: The project continues to use Node 20 (`node20` in `action.yaml`), which is fully compatible with these changes.

### Why these changes are needed:
- `@actions/core` v3 brings latest improvements and fixes from the GitHub Actions toolkit.
- Migrating to ESM is the modern standard for Node.js development and is required to consume ESM-only packages like the new `@actions/core` v3.

Verified with `npm test` and `npm run build`.

Reviewed-on: #73

__tests__/index.test.js

@@ -1,13 +1,13 @@
-const {
+import {
   validateAuthentication,
   replacePlaceholders,
   readReplaceAndWriteFiles,
   readFilesIntoDict,
-} = require("../index");
-const fs = require("fs");
-const path = require("path");
-const os = require("os");
-const { glob } = require("glob");
+} from "../index.js";
+import fs from "fs";
+import path from "path";
+import os from "os";
+import { glob } from "glob";
 
 describe("validateAuthentication", () => {
   it("should return null when only token is provided", () => {