Add a simple CLI tool for validating event schemas

[Zsailer]

Add a simple CLI powered Click and Rich to easily check if an authored schema is valid.

The colors from rich aren't rendering here, but they are pretty in the terminal :)

A valid schema looks like:

──────────────────────────────────── Validating the following schema ────────────────────────────────────

    {
      "$id": "http://event.jupyter.org/test",
      "version": 1,
      "title": "Simple Test Schema",
      "description": "A simple schema for testing\n",
      "type": "object",
      "properties": {
        "prop": {
          "title": "Test Property",
          "description": "Test property.",
          "type": "string"
        }
      }
    }

──────────────────────────────────────────────── Results ────────────────────────────────────────────────

✔ Nice work! This schema is valid.

And invalid schema looks like:

──────────────────────────────────── Validating the following schema ────────────────────────────────────

    {
      "$id": "http://event.jupyter.org/test",
      "version": 1,
      "title": "Simple Test Schema",
      "description": "A simple schema for testing\n",
      "type": "object",
      "properties": {
        "__badName": {
          "title": "Test Property",
          "description": "Test property.",
          "type": "string"
        }
      }
    }

──────────────────────────────────────────────── Results ────────────────────────────────────────────────
❌ The schema failed to validate.

We found the following error with your schema:

    '__badName' is an invalid property name because it starts with `__`. Properties starting with
    'dunder' are reserved as special meta-fields for Jupyter Events to use.