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 (28 specs total).

Bare-bracket rejection in strict2 (7 specs):

• {{ ['product'] }}, {{ [key] }} are rejected with parse_error
• Rejection applies in variables, for, if, case, assign

strict2 accepts qualified access (2 specs):

• product['title'] and dot notation remain valid

self keyword (18 specs):

• self['product'] resolves variables through the normal scope chain
• self[key] performs dynamic variable lookups (key from environment OR local assign)
• Works in all parsing modes (lax, strict, strict2)
• Local assigns are visible via self['product']
• self can be reassigned ({% assign self = 'value' %})
• Returns empty string for unknown keys (lax and strict2)
• Supports nested property access (self['product'].title)
• Composes with other expressions: a[ self[ 'b' ] ], self[self['key1']]
• Works inside for loop bodies and capture blocks
• self keyword still resolves to SelfDrop even when environment defines a self key
• QUIRK documented: self contains 'key' always returns false because contains calls include? and SelfDrop doesn't implement it — use self['key'] for definedness checks

Backwards compatibility (1 spec):

• Lax mode still allows bare-bracket access

Depends on:

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

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[karreiro]

Thank you for this PR, @aswamy!

Could we make sure we cover the scenarios outlined in this file (reference)?

Thanks again for the PR!