Add prettier; Move svelte-preprocess from dependencies to devDependencies

src/frontend/.prettierrc

@@ -0,0 +1,15 @@
+{
+  "trailingComma": "all",
+  "endOfLine": "lf",
+  "semi": true,
+  "tabWidth": 2,
+  "plugins": ["prettier-plugin-svelte"],
+  "overrides": [
+    {
+      "files": "*.svelte",
+      "options": {
+        "parser": "svelte"
+      }
+    }
+  ]
+}