Add type `RBS::Unnamed::TopLevelSelfClass` to core

[ksss]

First step of https://github.com/ruby/rbs/issues/2361

I defined RBS::Unnamed::TopLevelSelfClass.
By itself, this doesn't change anything, but if type checkers implement this class as the class of the top-level object, it will make it possible to type-check methods specific to the top level.