update two concurrent tasks to work on Windows, as '&' is not supported

package.json

@@ -17,7 +17,9 @@
     "build": "npm run build:js && npm run build:site",
     "build:js": "rollup -c --environment BUILD:production",
     "build:site": "eleventy --config .eleventy.js",
-    "start": "rollup -c -w & eleventy --serve --config .eleventy.js"
+    "start:build": "rollup -c -w",
+    "start:run": "eleventy --serve --config .eleventy.js",
+    "start": "npm-run-all --parallel start:*"
   },
   "devDependencies": {
     "@11ty/eleventy": "~1.0",
@@ -42,6 +44,7 @@
     "font-awesome": "https://git@github.com/FortAwesome/Font-Awesome.git#v4.7.0",
     "highlight.js": "^9.18.3",
     "highlightjs-line-numbers.js": "^2.8.0",
+    "npm-run-all": "^4.1.5",
     "postcss": "^8.4.6",
     "postcss-import": "^14.0.2",
     "postcss-preset-env": "^7.3.1",