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/strategyHelpers/blueGreen/promote.ts

@@ -1,13 +1,13 @@
 import * as core from '@actions/core'
 
-import {Kubectl} from '../../types/kubectl'
+import {Kubectl} from '../../types/kubectl.js'
 
-import {BlueGreenDeployment} from '../../types/blueGreenTypes'
-import {deployWithLabel, NONE_LABEL_VALUE} from './blueGreenHelper'
+import {BlueGreenDeployment} from '../../types/blueGreenTypes.js'
+import {deployWithLabel, NONE_LABEL_VALUE} from './blueGreenHelper.js'
 
-import {validateIngresses} from './ingressBlueGreenHelper'
-import {validateServicesState} from './serviceBlueGreenHelper'
-import {validateTrafficSplitsState} from './smiBlueGreenHelper'
+import {validateIngresses} from './ingressBlueGreenHelper.js'
+import {validateServicesState} from './serviceBlueGreenHelper.js'
+import {validateTrafficSplitsState} from './smiBlueGreenHelper.js'
 
 export async function promoteBlueGreenIngress(
    kubectl: Kubectl,