Support type narrowing

Ref: https://github.com/soutaro/steep/issues/472

Currently RBS doesn't have any syntax to narrow types in control flow.

So there's no way to achieve the following:

a = [1, nil].sample # a is `Integer | nil`
unless a.nil?
  # a is `Integer` here
  p a + 1
end

But actually, both Sorbet and Steep treat some special methods to narrow types

https://github.com/sorbet/sorbet/blob/718bc64f895abeeff88f56efbc92a3554ffaa4f2/infer/environment.cc#L536-L545

https://github.com/soutaro/steep/blob/a868762c2bd09f0954b05cdd9eab1819e14a51d3/lib/steep/interface/builder.rb#L709-L721

Ideally, I think we need to have a syntax considering Ruby gems which extend Object like ActiveSupport's present?.

I'm not very familiar with type system though, I guess the syntax would be similar with TypeScript's type predicates

https://www.typescriptlang.org/docs/handbook/2/narrowing.html#using-type-predicates

class Object[T?]
  def nil?: () -> self is nil
end

class Object[T?]
  def present?: () -> self is T
end

Start by reading the referenced Sorbet and Steep implementations, then inspect the RBS syntax and type-checking areas related to method signatures and control flow. The work is done when a reviewed syntax and behavior for type narrowing, including methods such as nil? and present?, is supported and tested.