Easier way to run Prettier locally (#1203)

Thanks, Ben Pickles - refs #1167

.github/workflows/prettier.yml

@@ -19,4 +19,4 @@ jobs:
       run: npm ci
     - name: Run prettier
       run: |-
-        npx --no-install prettier --check 'datasette/static/*[!.min].js'
+        npm run prettier -- --check

package.json

@@ -3,5 +3,9 @@
   "private": true,
   "devDependencies": {
     "prettier": "^2.2.1"
+  },
+  "scripts": {
+    "fix": "npm run prettier -- --write",
+    "prettier": "prettier 'datasette/static/*[!.min].js'"
   }
 }