Reject bare-bracket syntax in strict2 and introduce self keyword

Add bare-bracket rejection to Parser#expression in strict2 mode, so that
`['var']` is disallowed and `self['var']` is the required syntax.

- Add `Expression::SELF` constant ('self')
- Add `Parser#reject_bare_brackets` option, checked in `expression`
- Add `ParseContext#reject_bare_brackets?` and `force_reject_bare_brackets`
- Add `VariableLookupDrop` for `self['var']` scope-chain lookups
- Add `Variable#==` for rewriter state comparison
- Update `Context#find_variable` to return `VariableLookupDrop` for `self`

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

lib/liquid/expression.rb

@@ -2,6 +2,8 @@
 
 module Liquid
   class Expression
+    SELF = 'self'
+
     LITERALS = {
       nil => nil,
       'nil' => nil,