fix: handle Windows with .cmd/.ps1 shims and add tests

- Extract ensureAliasLinks to its own module for testability
- On Windows, create .cmd and .ps1 shims instead of symlinks
- On Unix, create symlinks (as before)
- Skip alias creation when targets don't exist (pnpm v10)
- Add vitest and 8 tests covering unix/windows/skip/no-overwrite

package.json

@@ -3,6 +3,7 @@
   "scripts": {
     "build:bundle": "esbuild src/index.ts --bundle --platform=node --target=node24 --format=cjs --minify --outfile=dist/index.js --loader:.json=json",
     "build": "pnpm run build:bundle",
+    "test": "vitest run",
     "start": "pnpm run build && sh ./run.sh"
   },
   "dependencies": {
@@ -18,6 +19,7 @@
   },
   "devDependencies": {
     "esbuild": "^0.27.4",
-    "typescript": "^5.3.3"
+    "typescript": "^5.3.3",
+    "vitest": "^4.1.2"
   }
 }