Adds https://github.com/11ty/eleventy/issues/386 to default eleventy-base-blog config

.eleventy.js

@@ -1,4 +1,5 @@
 const { DateTime } = require("luxon");
+const fs = require("fs");
 const pluginRss = require("@11ty/eleventy-plugin-rss");
 const pluginSyntaxHighlight = require("@11ty/eleventy-plugin-syntaxhighlight");
 
@@ -50,6 +51,20 @@ module.exports = function(eleventyConfig) {
     .use(markdownItAnchor, opts)
   );
 
+  eleventyConfig.setBrowserSyncConfig({
+    callbacks: {
+      ready: function(err, browserSync) {
+        const content_404 = fs.readFileSync('_site/404.html');
+
+        browserSync.addMiddleware("*", (req, res) => {
+          // Provides the 404 content without redirect.
+          res.write(content_404);
+          res.end();
+        });
+      }
+    }
+  });
+
   return {
     templateFormats: [
       "md",