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/utils.ts

@@ -1,9 +1,9 @@
 import * as core from '@actions/core'
 import * as fs from 'fs'
 import {KubeConfig} from '@kubernetes/client-node'
-import {getDefaultKubeconfig} from './kubeconfigs/default'
-import {getArcKubeconfig} from './kubeconfigs/arc'
-import {Cluster} from './types/cluster'
+import {getDefaultKubeconfig} from './kubeconfigs/default.js'
+import {getArcKubeconfig} from './kubeconfigs/arc.js'
+import {Cluster} from './types/cluster.js'
 
 /**
  * Gets the kubeconfig based on Kubernetes cluster type