Fix sourcemap generation

So, there is a bug in the parcel-bundler. I was generating the
sourcemap with a wrong path, but in reality it should've been
relative to the client code. I.e. my bundle in `/public/client.js`
but inside that file the last string is a link to the source maps
and it was `/client.js.map`, and of course Firefox was unable to
find that file.

parcel-bundler/parcel#1028

package.json

@@ -3,8 +3,8 @@
   "version": "1.0.0",
   "private": true,
   "scripts": {
-    "dev:client": "parcel watch src/client.js --out-dir public",
-    "live:client": "parcel build src/client.js --out-dir public",
+    "dev:client": "parcel watch src/client.js --out-dir public --public-url .",
+    "live:client": "parcel build src/client.js --out-dir public --public-url .",
     "lint": "prettier --check \"src/**/*.js\"",
     "fix": "prettier --write \"src/**/*.js\"",
     "start": "npm run dev:client & nodemon --watch src src/server.js"