Reject bare-bracket syntax in strict2 and introduce self keyword

[aswamy]

Summary

Adds bare-bracket rejection to the strict2 parser and introduces a self keyword for explicit variable lookups.

Parser changes:

• Parser#expression now raises a SyntaxError when it encounters bare-bracket access (['product']) in strict2/rigid mode
• ParseContext#new_parser passes reject_bare_brackets: true when error_mode is :strict2 or :rigid

self keyword:

• New Expression::SELF constant ('self')
• New SelfDrop class — a drop that provides variable-only access to the context's scope chain (local > file > global), without exposing filters, interrupts, or other context internals
• Context#find_variable returns a SelfDrop when the key is self and self hasn't been explicitly assigned as a local variable
• If self is explicitly assigned (e.g. {% assign self = 'value' %}), the local value takes precedence
• Context#variable_defined? — new method that checks key existence across scopes/environments using Hash#key?, so nil-valued variables are correctly treated as defined (used by SelfDrop#key?)
• Annotated with YARD @liquid_public_docs tags for public documentation generation

Variable#==:

• Added equality method comparing name and filters, used by the rewriter to detect AST equivalence when deciding whether to flag nodes for rewrite

Tophat

cd /path/to/liquid
bundle install
bin/render  # or: bundle exec irb -r liquid

Bare-bracket rejection in strict2:

# Rejected
Liquid::Template.parse("{{ ['product'] }}", error_mode: :strict2)
# => Liquid::SyntaxError: Bare bracket access is not allowed in strict2 mode. Use self['...'] instead

# Accepted
Liquid::Template.parse("{{ product }}", error_mode: :strict2)
Liquid::Template.parse("{{ self['product'] }}", error_mode: :strict2)
Liquid::Template.parse("{{ product['title'] }}", error_mode: :strict2)

self resolves through the scope chain:

# self['var'] walks local > file > global
t = Liquid::Template.parse("{{ self['product'] }}")
t.render('product' => 'shoes')
# => "shoes"

# Local assigns take precedence
t = Liquid::Template.parse("{% assign product = 'local' %}{{ self['product'] }}")
t.render('product' => 'global')
# => "local"

# Assigning self itself works
t = Liquid::Template.parse("{% assign self = 'hello' %}{{ self }}")
t.render
# => "hello"

# nil-valued variables are correctly handled under strict_variables
t = Liquid::Template.parse("{{ self['x'] }}")
t.render!({ 'x' => nil }, strict_variables: true)
# => "" (no UndefinedVariable error)

Lax mode is unaffected:

Liquid::Template.parse("{{ ['product'] }}", error_mode: :lax)
# => OK, no error

🤖 Generated with Claude Code

[graygilmore]

The drop looks good to me but I'd prefer to have somebody with a little more Liquid context give another approval.