Migrate to ESM with esbuild/vitest and upgrade to node24 (#260)

- 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)

src/index.ts

@@ -1,4 +1,4 @@
-import {run} from './run'
+import {run} from './run.js'
 import * as core from '@actions/core'
 
 run().catch(core.setFailed)