Add T::Enum support

[dduugg]

YARD currently skips over the enums block. Thus, to use the Sorbet docs example, this code:

class Suit < T::Enum
  enums do
    Spades = new
    Hearts = new
    Clubs = new
    Diamonds = new
  end
end

resulds in the following YARD documentation:

We should add a handler that displays the values defined in the enums block.