ci: add a prettier check

Prettier becomes a devDependency with a lockfile, so the check is
reproducible: npm ci installs the pinned version and
npm run format:check enforces it. Markdown stays outside Prettier's
scope through .prettierignore, since its prose and table reflow churns
docs without improving consistency. The check runs on the same
triggers as the test and benchmark jobs.

Signed-off-by: Bryan English <bryan@bryanenglish.com>

Author: bengl

.github/workflows/ci.yml

@@ -32,3 +32,13 @@ jobs:
         with:
           node-version: latest
       - run: npm run bench
+
+  prettier:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v7
+      - uses: actions/setup-node@v7
+        with:
+          node-version: latest
+      - run: npm ci
+      - run: npm run format:check

.gitignore

@@ -0,0 +1 @@
+node_modules/

.prettierignore

@@ -0,0 +1 @@
+*.md

package-lock.json

@@ -27,6 +27,10 @@
   },
   "scripts": {
     "test": "node --test 'test/test-*.js'",
-    "bench": "node --experimental-bench --bench 'benchmark/*.bench.js'"
+    "bench": "node --experimental-bench --bench 'benchmark/*.bench.js'",
+    "format:check": "prettier --check ."
+  },
+  "devDependencies": {
+    "prettier": "3.9.10"
   }
 }