Migrate to ESM with esbuild/vitest and upgrade to node24

- Add "type": "module" to package.json for ESM support
- Replace ncc with esbuild for bundling (with createRequire banner for CJS compat)
- Replace jest/ts-jest with vitest for testing
- Upgrade @actions/core to ^3.0.0, @actions/exec to ^3.0.0, @actions/io to ^3.0.2, @actions/tool-cache to 4.0.0 (ESM-only versions)
- Update tsconfig.json to use NodeNext module resolution
- Convert all tests to use vi.mock() factory pattern for ESM-only module compatibility
- Update action.yml runtime from node20 to node24
- Bump version from 4.3.1 to 5.0.0 (major version bump for breaking node runtime change)

vitest.config.ts

@@ -0,0 +1,10 @@
+import {defineConfig} from 'vitest/config'
+
+export default defineConfig({
+   test: {
+      globals: true,
+      environment: 'node',
+      include: ['**/*.test.ts'],
+      clearMocks: true
+   }
+})