[Writer] Preserve an empty line between declarations and members

[pocke]

By this change, the writer will preserve an empty line between declarations and members.

I also added a smoke test for Ruby::Signature::Writer.
It only checks the writer generates the same meaning signature.

Format examples

no changes

class A
  def foo: () -> void
  def bar: () -> void
end
class B
end

also no changes

class A
  def foo: () -> void

  def bar: () -> void
end

class B
end

But it removes too many empty lines. the following code will be formatted to the same as the previous example.

class A
  def foo: () -> void


  def bar: () -> void
end


class B
end

[soutaro]

👍

[soutaro]

@pocke Thanks!