Add support for require plural field name for lists

[mmeinzer]

It would be nice to have a rule for enforcing plural naming of list fields - possibly list-field-name-grammatical-number. Below are some examples. Non-list fields would be entirely ignored by this rule.

With { style: "singular" }

// good
model User {
  email String[]
}

// bad
model Users {
  email String[]
}

With { style: "plural" }

// good
model User {
  emails String[]
}

// bad
model Users {
  email String[]
}

[mmeinzer]

Happy to put up a PR for this if there's interest