chore: make the npm run dev:site command executable on windows

.gitignore

@@ -3,5 +3,4 @@
 _site/
 dist/
 node_modules/
-package-lock.json
 src/compiled-assets

package.json

@@ -9,7 +9,7 @@
     "del:dist": "rimraf ./dist",
     "dev": "npm run dev:assets & npm run dev:site",
     "dev:assets": "webpack --config webpack.config.dev.js",
-    "dev:site": "ELEVENTY_ENV=development npx eleventy --serve",
+    "dev:site": "cross-env ELEVENTY_ENV=development npx eleventy --serve",
     "prod": "npm-run-all del:dist del:assets build:assets build:site",
     "serve:prod": "serve ./dist/"
   },
@@ -51,6 +51,7 @@
     "webpack-merge": "^5.8.0"
   },
   "devDependencies": {
+    "cross-env": "^7.0.3",
     "prettier": "^2.0.5"
   }
 }