Add an easier syntax to create methods and signatures

[Morriar]

Creating a new method with a signature can be a bit verbose when manipulating the RBI classes directly:

tree << RBI::Method.new("foo") do |node|
  node << RBI::ReqParam.new("p1")
  node << RBI::OptParam.new("p2", "42")

  node.sigs << RBI::Sig.new do |sig|
    sig << RBI::SigParam.new("p1", "T.untyped")
    sig << RBI::SigParam.new("p2", "Integer")
  end
end

This PR introduces a more elegant syntax:

tree << RBI::Method.new("foo") do |node|
  node.add_param("p1")
  node.add_opt_param("p2", "42")

  node.add_sig do |sig|
    sig.add_param("p1", "T.untyped")
    sig.add_param("p2", "Integer")
  end
end

[andyw8]

Since these are all in the form add_..., perhaps it could be shortened to param, opt_param etc? No big deal though.

[Morriar]

The issue is with add_sig, defining a method named sig in a Sorbet project is creating all kind of trouble. So I went with add_ prefix everywhere

[paracycle]

🙏