Add per-spec required_features support

[tobi]

Summary

Adds support for individual specs to declare required_features, allowing them to be skipped when the adapter doesn't support those features.

Changes

• Add required_features field to Unit struct
• Add runnable_with? and missing_features methods to Unit
• Filter specs by required_features in the runner
• Add shopify_error_handling feature for Shopify-specific error behavior
• Move test_subcontext_enter_fails to error_handling.yml with required_features: [shopify_error_handling]

Why

The test_subcontext_enter_fails_and_leaves_runtime_in_good_state test expects specific Shopify-internal error behavior (Liquid error (line 2): internal) that differs from the open source implementation. The open source Liquid correctly renders the template, but Shopify's internal version uses a custom context class that forces errors.

By adding per-spec required_features, we can skip this test for adapters that don't implement Shopify's internal error handling while still running it for Shopify's internal implementation.

Example usage in YAML

- name: some_test
  required_features:
    - shopify_error_handling
  template: "..."
  expected: "..."