Restore markdown-it script to browser demo to support "?renderer=markdown-it".

Author: DavidAnson

demo/default.htm

@@ -33,6 +33,7 @@
       <footer>Copyright &copy; <a href="https://dlaa.me/">David Anson</a></footer>
     </div>
   </div>
+  <script src="markdown-it.min.js"></script>
   <script src="micromark-browser.js"></script>
   <script src="micromark-html-browser.js"></script>
   <script src="markdownlint-browser.min.js"></script>

package.json

@@ -27,7 +27,7 @@
     "build-config-example": "node schema/build-config-example.js",
     "build-config-schema": "node schema/build-config-schema.js",
     "build-declaration": "tsc --allowJs --declaration --emitDeclarationOnly --module commonjs --outDir dts --resolveJsonModule --target es2015 lib/markdownlint.js && node scripts copy dts/lib/markdownlint.d.ts lib/markdownlint.d.ts && node scripts remove dts",
-    "build-demo": "node scripts copy node_modules/markdownlint-micromark/micromark-browser.js demo/micromark-browser.js && node scripts copy node_modules/markdownlint-micromark/micromark-html-browser.js demo/micromark-html-browser.js && cd demo && webpack --no-stats",
+    "build-demo": "node scripts copy node_modules/markdown-it/dist/markdown-it.min.js demo/markdown-it.min.js && node scripts copy node_modules/markdownlint-micromark/micromark-browser.js demo/micromark-browser.js && node scripts copy node_modules/markdownlint-micromark/micromark-html-browser.js demo/micromark-html-browser.js && cd demo && webpack --no-stats",
     "build-docs": "node doc-build/build-rules.mjs",
     "build-example": "npm install --no-save --ignore-scripts grunt grunt-cli gulp through2",
     "build-micromark": "cd micromark && npm run build",