feat: load js from cdn using Netlify's reverse proxy

- https://www.netlify.com/docs/redirects/

source/_redirects

@@ -2,3 +2,6 @@
 / /blog/ 302
 # Redirect post to /blog/
 /:year/:month/:date/:slug /blog/:year/:month/:date/:slug/ 301
+
+# Reverse proxy to cdn
+/extjs/* https://cdnjs.cloudflare.com/ajax/libs/:splat 200

themes/typing/layout/_partial/head.ejs

@@ -52,4 +52,6 @@
   <% if (theme.icons) { %>
     <%- addCss({ href: 'https://cdn.statically.io/libs/fork-awesome/1.1.7/css/fork-awesome.min.css', 'data-sri-fallback': '/forkawesome/css/fork-awesome.min.css', integrity: 'sha384-mByhW6NjnxyShh67P9+fepUvYSd7Uz/qV6e2u4kA2Fi4ZkjXxIP2mRkyK9dwK24W', crossorigin: 'anonymous' }) %>
   <% } %>
+
+  <%- addJs('/extjs/jquery/3.4.1/jquery.slim.js') %>
 </head>