_headers & _redirects generation (#240)

* Generate `_headers` and `_redirects` by passing assets through ejs templates.

* PR feedback

* Excluding service worker stuff from prerender

* Build SW in dev

* Let's give this a try

* lol

* Is this how it works?

src/lib/offliner.ts

@@ -42,6 +42,9 @@ async function updateReady(reg: ServiceWorkerRegistration): Promise<void> {
 
 /** Set up the service worker and monitor changes */
 export async function offliner(showSnack: SnackBarElement['showSnackbar']) {
+  // This needs to be a typeof because Webpack.
+  if (typeof PRERENDER === 'boolean') return;
+
   if (process.env.NODE_ENV === 'production') {
     navigator.serviceWorker.register('../sw');
   }