Add sig/manifest.yaml to declare stdlib dependencies

[tk0miya]

sig/herb/token_list.rbs declares a class inheriting from SimpleDelegator, which comes from the delegate standard library. Since delegate is a default gem, it does not appear in the gemspec, and rbs collection install cannot resolve it on the consumer side.

With Steep 2.0, this previously-silent issue surfaces as a hard error:

Type checking failed due to error in library RBS file
Cannot find type `SimpleDelegator`
(.../herb-0.10.1/sig/herb/token_list.rbs:4:2)
Diagnostic ID: Ruby::LibraryRBSError

Declare the implicit standard library dependencies in sig/manifest.yaml so downstream users get correct type resolution.

Refs: https://github.com/ruby/rbs/blob/master/docs/gem.md

[marcoroth]

Thank you @tk0miya! 🙏🏼

Is there a way we can check in CI that Herb is bundling/exposing the RBS signatures properly so we don't introduce any regressions?

[tk0miya]

@marcoroth we can check the validity of types via rbs validate command. As a trial, I implemented the validation on https://github.com/marcoroth/herb/compare/main...tk0miya:herb:rbs_validate?expand=1.

At present, some serialized_* type aliases are not defined. So we need to define them before validating them on CI.

[marcoroth]

Awesome, thank you for investigating @tk0miya!

If we move the bundle exec rbs validate command to the existing build.yml we should also have the serialized_* types available as they are getting generated automatically from the ERB files in templates/ via the rake templates command here: https://github.com/marcoroth/herb/blob/main/templates/sig/serialized_ast_nodes.rbs.erb