Add $schema as expected property to config

[nivekithan]

This pr adds $schema as expected property to config so that parsing the following json file will not cause any error

{
  "$schema": "https://www.schemastore.org/sqlc-2.0.json",
  "version": "2",
  "sql": [
    {
      "engine": "sqlite",
      "queries": "./problems/means-to-end/sql/queries.sql",
      "schema": "./problems/means-to-end/sql/schema.sql",
      "gen": {
        "go": {
          "package": "db",
          "out": "./problems/means-to-end/db"
        }
      }
    }
  ]
}

Result

Change when running the generate command for the above config file

Before

sqlc generate
error parsing sqlc.json: yaml: unmarshal errors:
  line 2: field $schema not found in type config.Config

After

sqlc-dev generate

# exited without any error