Update Jest and TS config for ESM support (#465) (#466)

Refactor configuration to support ESM-only @actions packages:
- Add "type": "module" to package.json for ESM
- Update tsconfig.json to use NodeNext module resolution
- Convert jest.config.js to ESM with ts-jest ESM preset
- Replace ncc with rollup for bundling
- Update source imports to use .js extensions for ESM compatibility

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: JKRT

__tests__/installer.test.ts

@@ -1,9 +1,9 @@
-import * as installer from '../src/installer'
+import * as installer from '../src/installer.js'
 import * as exec from '@actions/exec'
 
 import * as fs from 'fs'
 import * as os from 'os'
-import { expect, test } from '@jest/globals'
+import {expect, test} from '@jest/globals'
 
 const osPlat = os.platform()