Add singular resource routing and specs

[anuj-pal27]

Summary

Implements the singular resource route helper in the router DSL and updates the spec suite to validate all supported options and behaviors.

Changes

• Add resource helper routes for singular resources.
• Include GET /:resource/new in the default action set.
• Respect only / except filters.
• Ignore param: for singular resources (but raise on invalid values).
• Update specs for modules, paths, scopes, nesting, and multiple resources.

Testing

• bundle exec rubocop spec/router/dsl_spec.rb
• bundle exec rspec spec/router/dsl_spec.rb

fix #119