fix: reject abstract singleton methods on interfaces

[Saithej2k]

Summary

• Reject abstract singleton method declarations on interfaces
• Cover both abstract def self.foo and class << self; abstract def foo; end forms
• Remove the just-defined singleton method stub before raising

Closes #5

Validation

• git diff --check

Not run locally: test suite requires Ruby 3.4.3; the local system Ruby is 2.6.10.