Specify specific prettier version (#186)

* Add explicit prettier version

* Update CONTRIBUTING.md

* Add .prettierrc.json file to ensure we always use es5 trailing commas

web/.prettierrc.json

@@ -0,0 +1,3 @@
+{
+  "trailingComma": "es5"
+}

web/package.json

@@ -30,5 +30,8 @@
     "tailwindcss": "^3.3.1",
     "typescript": "5.0.3",
     "yup": "^1.1.1"
+  },
+  "devDependencies": {
+    "prettier": "2.8.8"
   }
 }