Migrate to ESM with esbuild, vitest (#227)

* Migrate to ESM, esbuild, vitest, and update actions/* to latest

- Update @actions/core to 3.x, @actions/exec to 3.x, @actions/io to 3.x
- Replace @vercel/ncc + babel with esbuild (build.mjs) targeting Node 20 ESM
- Replace jest/babel-jest/ts-jest with vitest and @vitest/coverage-v8
- Update tsconfig to NodeNext module resolution with strict mode
- Add explicit .js extensions to all relative imports (NodeNext requirement)
- Fix implicit any index signatures in parseCluster and parseMethod
- Migrate all test files from jest to vi.mock/vi.mocked APIs
- Fix ESM module spying limitations using vi.mock() at module level
- Fix env var test pollution in default.test.ts with afterEach cleanup

* remove build.mjs and update build script

* update pkg lock

* update pkg lock

src/action.test.ts

@@ -1,7 +1,9 @@
-import {getRequiredInputError} from '../tests/util'
-import {run} from './action'
-import fs from 'fs'
-import * as utils from './utils'
+import {vi, describe, it, expect} from 'vitest'
+import {getRequiredInputError} from '../tests/util.js'
+import {run} from './action.js'
+
+vi.mock('fs')
+vi.mock('./utils.js')
 
 describe('Run', () => {
    it('throws error without cluster type', async () => {
@@ -9,22 +11,23 @@ describe('Run', () => {
    })
 
    it('writes kubeconfig and sets context', async () => {
+      const {getKubeconfig, setContext} = await import('./utils.js')
+      const fs = await import('fs')
       const kubeconfig = 'kubeconfig'
 
       process.env['INPUT_CLUSTER-TYPE'] = 'default'
       process.env['RUNNER_TEMP'] = '/sample/path'
 
-      jest
-         .spyOn(utils, 'getKubeconfig')
-         .mockImplementation(async () => kubeconfig)
-      jest.spyOn(fs, 'writeFileSync').mockImplementation(() => {})
-      jest.spyOn(fs, 'chmodSync').mockImplementation(() => {})
-      jest.spyOn(utils, 'setContext').mockImplementation(() => kubeconfig)
+      vi.mocked(getKubeconfig).mockResolvedValue(kubeconfig)
+      vi.mocked(setContext).mockReturnValue(kubeconfig)
+      vi.mocked(fs.writeFileSync).mockImplementation(() => {})
+      vi.mocked(fs.chmodSync).mockImplementation(() => {})
 
-      expect(await run())
-      expect(utils.getKubeconfig).toHaveBeenCalled()
+      await run()
+
+      expect(getKubeconfig).toHaveBeenCalled()
       expect(fs.writeFileSync).toHaveBeenCalled()
       expect(fs.chmodSync).toHaveBeenCalled()
-      expect(utils.setContext).toHaveBeenCalled()
+      expect(setContext).toHaveBeenCalled()
    })
 })