Install Prettier via package.json (#1170)

* Error if Prettier isn't already installed
* Temporarily run Prettier check on every commit
* Install and run Prettier via package.json
* Trigger another prettier check on CI

.github/workflows/prettier.yml

@@ -1,9 +1,6 @@
 name: Check JavaScript for conformance with Prettier
 
-on:
-  push:
-    paths:
-    - 'datasette/static/*'
+on: [push]
 
 jobs:
   prettier:
@@ -15,9 +12,11 @@ jobs:
       name: Configure npm caching
       with:
         path: ~/.npm
-        key: ${{ runner.os }}-npm-${{ hashFiles('**/workflows/prettier.yml') }}
+        key: ${{ runner.OS }}-npm-${{ hashFiles('**/package-lock.json') }}
         restore-keys: |
-          ${{ runner.os }}-npm-
+          ${{ runner.OS }}-npm-
+    - name: Install dependencies
+      run: npm ci
     - name: Run prettier
       run: |-
-        npx prettier --check 'datasette/static/*[!.min].js'
+        npx --no-install prettier --check 'datasette/static/*[!.min].js'