Bump @actions/core from 1.10.1 to 3.0.0 and @actions/exec from 1.1.1 to 3.0.0

Both packages moved to ESM-only in v3, so this migrates the project
from CommonJS to ESM: adds "type": "module" to package.json, switches
tsconfig to module/moduleResolution "nodenext", adds .js extensions to
all relative imports, and reconfigures Jest for ESM support.

package.json

@@ -5,6 +5,7 @@
     "node": ">= 24"
   },
   "private": true,
+  "type": "module",
   "description": "Execute your workflow steps only if relevant files are modified.",
   "main": "lib/main.js",
   "scripts": {
@@ -13,7 +14,7 @@
     "format-check": "prettier --check **/*.ts",
     "lint": "eslint src/**/*.ts",
     "pack": "ncc build",
-    "test": "jest",
+    "test": "NODE_OPTIONS='--experimental-vm-modules' jest",
     "all": "npm run build && npm run format && npm run lint && npm run pack && npm test"
   },
   "repository": {
@@ -28,8 +29,8 @@
   "author": "YourNameOrOrganization",
   "license": "MIT",
   "dependencies": {
-    "@actions/core": "^2.0.3",
-    "@actions/exec": "^2.0.0",
+    "@actions/core": "^3.0.0",
+    "@actions/exec": "^3.0.0",
     "@actions/github": "^8.0.1",
     "picomatch": "^4.0.3"
   },