Associate singleton methods with the right owner objects

[vinistock]

Motivation

First step for #1939

This PR starts indexing singleton contexts and associating entities to singleton owners.

Next up is fixing the ancestor of singleton classes, which take a few more things into account than what we're currently doing.

Implementation

The idea is to maintain the same approach we've been using for constants so far because everything else works by default if we do it (constant, ivar and method resolutions and lookups).

We essentially use a special name to declare the singleton namespace, which matches somewhat what Ruby does <Class:Name>. It works exactly like any other namespace.

This makes it easy to resolve and lookup things. We can simply build the exact same nesting structure on the language server and then pass those namespaces for resolution.

Automated Tests

I added tests demonstrating that resolution for methods, constants and ivars already work by default by following this approach.

[paracycle]

The changes look good to me, but let's make sure to test that given:

class Foo
  class << self
    def self.foo
    end
  end
end

the owner of the foo method is <Class:<Class:Foo>>.

Nothing else seems to be exercising the when Prism::SelfNode case inside a singleton class.