Migrate to esbuild/Vitest and upgrade @actions/* to ESM-only versions (#492)

* Migrate build toolchain from ncc/Jest to esbuild/Vitest

Replace the legacy ncc/Jest/Babel build stack with a modern ESM toolchain:

Build:
- Replace @vercel/ncc with esbuild (--platform=node --target=node20 --format=esm)
- Add createRequire banner for CJS interop in ESM bundle
- Add "type": "module" to package.json
- Add tsc --noEmit typecheck script (esbuild strips types without checking)
- Add typecheck to husky pre-commit hook

Dependencies:
- Bump @actions/core@3, exec@3, io@3, tool-cache@4 (ESM-only)
- Replace jest/ts-jest/@babel/* with vitest@4

Tests:
- Convert 29 test files: jest.fn()→vi.fn(), jest.mock()→vi.mock(), jest.spyOn()→vi.spyOn()
- Fix vitest 4 compat: mockImplementation requires args, mock call tracking, await .rejects

CI:
- Update build step from ncc build → npm run build
- Update composite action to use npm run build

* Switch tsconfig to NodeNext module resolution

Change module/moduleResolution from ES2022/bundler to NodeNext/NodeNext
and target from ES2022 to ES2020.

- Add .js extensions to all relative imports across 59 source/test files
  (required by NodeNext module resolution)
- Add vitest/globals to tsconfig types array for global test API declarations

src/run.ts

@@ -1,19 +1,19 @@
 import * as core from '@actions/core'
-import {getKubectlPath, Kubectl} from './types/kubectl'
+import {getKubectlPath, Kubectl} from './types/kubectl.js'
 import {
    deploy,
    ResourceTypeFleet,
    ResourceTypeManagedCluster
-} from './actions/deploy'
-import {ClusterType} from './inputUtils'
-import {promote} from './actions/promote'
-import {reject} from './actions/reject'
-import {Action, parseAction} from './types/action'
-import {parseDeploymentStrategy} from './types/deploymentStrategy'
-import {getFilesFromDirectoriesAndURLs} from './utilities/fileUtils'
-import {PrivateKubectl} from './types/privatekubectl'
-import {parseResourceTypeInput} from './inputUtils'
-import {parseDuration} from './utilities/durationUtils'
+} from './actions/deploy.js'
+import {ClusterType} from './inputUtils.js'
+import {promote} from './actions/promote.js'
+import {reject} from './actions/reject.js'
+import {Action, parseAction} from './types/action.js'
+import {parseDeploymentStrategy} from './types/deploymentStrategy.js'
+import {getFilesFromDirectoriesAndURLs} from './utilities/fileUtils.js'
+import {PrivateKubectl} from './types/privatekubectl.js'
+import {parseResourceTypeInput} from './inputUtils.js'
+import {parseDuration} from './utilities/durationUtils.js'
 
 export async function run() {
    // verify kubeconfig is set