Add svelte settings to .prettierrc (#173)

Author: dimfeld

.prettierrc

@@ -1,5 +1,14 @@
 {
   "singleQuote": true,
   "printWidth": 100,
-  "trailingComma": "es5"
+  "trailingComma": "es5",
+  "plugins": ["prettier-plugin-svelte"],
+  "overrides": [
+    {
+      "files": "*.svelte",
+      "options": {
+        "parser": "svelte"
+      }
+    }
+  ]
 }