Add specs for bare-bracket rejection in strict2 and self keyword

[aswamy]

Summary

Adds conformance specs for the bare-bracket syntax changes in Shopify/liquid:

Bare-bracket rejection in strict2 (7 specs):

• {{ ['product'] }}, {{ [key] }} are rejected with parse_error
• Rejection applies in variables, for, if, case, assign
• Qualified bracket access (product['title']) and dot notation remain valid

self keyword (8 specs):

• self['product'] resolves variables through the normal scope chain
• self[key] performs dynamic variable lookups
• Works in all parsing modes (lax, strict, strict2)
• Local assigns ({% assign product = 'local' %}) are visible via self['product']
• self can be reassigned ({% assign self = 'value' %})
• Returns empty string for unknown keys
• Supports nested property access (self['product'].title)

Backwards compatibility (1 spec):

• Lax mode still allows bare-bracket access

Depends on:

• Shopify/liquid: #2060
• Shopify/liquid-rewriter-private: #211

🤖 Generated with Claude Code