[Feature request] Introduce constant type

When I've recently opened PR to add signatures of the Parallel gem on the ruby/gem_rbs_collection repository, I encounter the following difficulty:

# Ruby code
items = [1, 2, 3]
Parallel.all?(-> { items.pop || Parallel::Stop }) { |item| item.size == 1 }

# RBS (short version)
module Parallel
  Stop: Object

  # `T | Stop` should be better than `T | untyped`.
  def self.all?: [T] (Enumerable[T] | (^() -> T | untyped) array) { (T) -> boolish } -> bool
end

As you see above, we now cannot use Parallel::Stop as a type.

Supporting such a type signature might be useful, but what do you think?

This proposal is thanks to @pocke. Please refer to https://github.com/ruby/gem_rbs_collection/pull/21#discussion_r587286192 for details.

Start with the issue's Parallel example and the linked gem_rbs_collection discussion to understand how a constant such as Parallel::Stop is currently parsed and represented. Determine the design needed for using that constant as a type, then add coverage for the example and verify that the signature can use Parallel::Stop instead of untyped.