Add CI step to validate examples against the JSON schema (#301)

Add a CI step to validate that the example migrations are valid
according to the JSON schema.

This would have caught the problem where the JSON schema was not fully
updated in #293 and had to be
updated in a follow-up PR #300
despite #293 including an example migration.

Use [ajv-cli](https://ajv.js.org/packages/ajv-cli.html) to perform
schema validation.

Here is an [example
run](https://github.com/xataio/pgroll/actions/runs/8113707179/job/22177613982)
where the check fails.

.github/workflows/build.yml

@@ -142,6 +142,10 @@ jobs:
       with:
         go-version-file: 'go.mod'
 
+    - name: Validate examples against JSON schema
+      run: |
+        npx -y ajv-cli --spec=draft2020 validate -s schema.json -d "./examples/*.json"
+
     - name: Run example migrations
       run: |
         if [ "$PGROLL_SCHEMA" != "public" ]; then