Support custom members of Data class and Struct class

[tk0miya]

Data class and Struct class can define custom members via block:

Measure = Data.define(:amount, :unit) do
  def <=>(other)
    return unless other.is_a?(self.class) && other.unit == unit
    amount <=> other.amount
  end
    
  include Comparable
end

This generates type definitions for these custom members.

refs:

• https://ruby-doc.org/3.4.1/Data.html
• https://ruby-doc.org/3.4.1/Struct.html