New rubocop cop to enforce our GraphQL Enum standards

We want to have a new cop that can enforce our GraphQL Enum standards (see &1838 for standards).

The cop would ensure our GraphQL Enums:

• have classes that end in Enum
• have graphql_names that do not include the word enum in any case
• have uppercase values

There are limits when it comes to enforcing uppercase values. The cop could identify these violations:

value 'foo'
value 'Foo'

But we would need GraphQL reviewers to be able to pick up whether this was a violation or not:

value foo