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

package-lock.json

@@ -8,7 +8,7 @@
       "name": "screeps-deploy-action",
       "version": "0.1.1",
       "dependencies": {
-        "@actions/core": "^2.0.0",
+        "@actions/core": "^3.0.0",
         "glob": "^13.0.0",
         "screeps-api": "^1.7.2"
       },
@@ -19,28 +19,28 @@
       }
     },
     "node_modules/@actions/core": {
-      "version": "2.0.3",
-      "resolved": "https://registry.npmjs.org/@actions/core/-/core-2.0.3.tgz",
-      "integrity": "sha512-Od9Thc3T1mQJYddvVPM4QGiLUewdh+3txmDYHHxoNdkqysR1MbCT+rFOtNUxYAz+7+6RIsqipVahY2GJqGPyxA==",
+      "version": "3.0.0",
+      "resolved": "https://registry.npmjs.org/@actions/core/-/core-3.0.0.tgz",
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       "license": "MIT",
       "dependencies": {
-        "@actions/exec": "^2.0.0",
-        "@actions/http-client": "^3.0.2"
+        "@actions/exec": "^3.0.0",
+        "@actions/http-client": "^4.0.0"
       }
     },
     "node_modules/@actions/exec": {
-      "version": "2.0.0",
-      "resolved": "https://registry.npmjs.org/@actions/exec/-/exec-2.0.0.tgz",
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+      "resolved": "https://registry.npmjs.org/@actions/exec/-/exec-3.0.0.tgz",
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       "license": "MIT",
       "dependencies": {
-        "@actions/io": "^2.0.0"
+        "@actions/io": "^3.0.2"
       }
     },
     "node_modules/@actions/http-client": {
-      "version": "3.0.2",
-      "resolved": "https://registry.npmjs.org/@actions/http-client/-/http-client-3.0.2.tgz",
-      "integrity": "sha512-JP38FYYpyqvUsz+Igqlc/JG6YO9PaKuvqjM3iGvaLqFnJ7TFmcLyy2IDrY0bI0qCQug8E9K+elv5ZNfw62ZJzA==",
+      "version": "4.0.0",
+      "resolved": "https://registry.npmjs.org/@actions/http-client/-/http-client-4.0.0.tgz",
+      "integrity": "sha512-QuwPsgVMsD6qaPD57GLZi9sqzAZCtiJT8kVBCDpLtxhL5MydQ4gS+DrejtZZPdIYyB1e95uCK9Luyds7ybHI3g==",
       "license": "MIT",
       "dependencies": {
         "tunnel": "^0.0.6",
@@ -48,9 +48,9 @@
       }
     },
     "node_modules/@actions/io": {
-      "version": "2.0.0",
-      "resolved": "https://registry.npmjs.org/@actions/io/-/io-2.0.0.tgz",
-      "integrity": "sha512-Jv33IN09XLO+0HS79aaODsvIRyduiF7NY/F6LYeK5oeUmrsz7aFdRphQjFoESF4jS7lMauDOttKALcpapVDIAg==",
+      "version": "3.0.2",
+      "resolved": "https://registry.npmjs.org/@actions/io/-/io-3.0.2.tgz",
+      "integrity": "sha512-nRBchcMM+QK1pdjO7/idu86rbJI5YHUKCvKs0KxnSYbVe3F51UfGxuZX4Qy/fWlp6l7gWFwIkrOzN+oUK03kfw==",
       "license": "MIT"
     },
     "node_modules/@babel/helper-string-parser": {