Make blank/empty comparisons invariant to ActiveSupport

- Implement liquid_blank? and liquid_empty? methods in Condition
  to emulate ActiveSupport's behavior when it's not loaded
- This ensures templates like `{% if x == blank %}` work identically
  whether ActiveSupport is loaded or not
- Update liquid-spec adapters for new API (ctx parameter)
- Add rake spec task for running liquid-spec matrix

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Gemfile

@@ -33,6 +33,6 @@ end
 
 group :spec do
   # Using feature branch until https://github.com/Shopify/liquid-spec/pull/97 is merged
-  gem 'liquid-spec', github: 'Shopify/liquid-spec', branch: 'add-per-spec-required-features'
+  gem 'liquid-spec', github: 'Shopify/liquid-spec', branch: 'main'
   gem 'activesupport', require: false
 end