Introduce NestTopLevelMembers rewriter

[Morriar]

RBS doesn't support method definitions at the top level:

rbs = RBS::Buffer.new(content: <<~RBS, name: "")
  def foo: -> void
RBS

RBS::Parser.parse_signature(rbs) # => Syntax error: cannot start a declaration, token=`def` (kDEF)

A trick we can play is to always rewrite the RBI file to nest all the top level members into an Object class so this:

def foo; end
attr_reader :bar

is rewritten to:

class Object
  def foo; end
  attr_reader :bar
end