Migrate to ESM with esbuild and vitest (#243)

* Migrate to ESM with esbuild, vitest and upgrade @actions/* to ESM-only versions

* fix: update lockfile for npm ci compatibility

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Co-authored-by: Suneha Bose <suneha.bose@gmail.com>

vitest.config.ts

@@ -0,0 +1,10 @@
+import {defineConfig} from 'vitest/config'
+
+export default defineConfig({
+   test: {
+      globals: true,
+      environment: 'node',
+      include: ['**/*.test.ts'],
+      clearMocks: true
+   }
+})