JSON files should be validated according to the schema file automatically on every PR

As discussed with @UmarMohamedUsman we have an [existing schema definition](https://github.com/Azure/review-checklists/blob/main/checklists/checklist.schema.json) which tells us the fields that are required in order for the JSON to be valid. It would be great to reference the schema in the JSON files (and drive VS Code IntelliSense probably? 🤔)... or at least have the JSON checked on every PR similar to the linting check.

Start by reading checklists/checklist.schema.json and inspecting the repository’s existing linting check for how pull-request validation is configured. Ensure the JSON files are checked against the schema on every PR, or reference the schema for editor support, and verify the resulting PR check catches invalid required fields.

Written by the indexing model from the issue text.