Update .gitignore in pwa example (vercel#44391)

The files created when running the app using next-pwa should not be added to git, so I added them to `.gitignore`.

Updated by referring to https://github.com/shadowwalker/next-pwa/blob/master/.gitignore.

examples/progressive-web-app/.gitignore

@@ -34,3 +34,15 @@ yarn-error.log*
 # typescript
 *.tsbuildinfo
 next-env.d.ts
+
+# next-pwa
+/public/precache.*.*.js
+/public/sw.js
+/public/workbox-*.js
+/public/worker-*.js
+/public/fallback-*.js
+/public/precache.*.*.js.map
+/public/sw.js.map
+/public/workbox-*.js.map
+/public/worker-*.js.map
+/public/fallback-*.js