[WIP] Add support for migrating liquid code away from several lax parse quirks

It transforms liquid code to equivalently parsed code, that doesn't depend
on lax parsing quirks.

Note that this transformation can hide the intent of the original code, so
it is best to review the changes, since removed syntax often would indicate
a bug in the original liquid code.

lib/liquid/variable_lookup.rb

@@ -10,6 +10,41 @@ module Liquid
       new(markup)
     end
 
+    LITERALS = Expression::LITERALS.keys.freeze
+    private_constant :LITERALS
+
+    def self.lax_migrate(markup)
+      new_markup = nil
+      last_match = nil
+      first_match = nil
+      markup.scan(VariableParser) do |lookup|
+        last_match = Regexp.last_match
+        first_match ||= last_match
+        if lookup&.start_with?('[') && lookup&.end_with?(']')
+          new_markup ||= +""
+          new_markup << "[" << Expression.lax_migrate(lookup[1..-2]) << "]"
+        else
+          new_markup << "." if new_markup
+          new_markup ||= +""
+          new_markup << lookup
+        end
+      end
+
+      case new_markup
+      when nil
+        # `markup.scan(VariableParser)` may skip over all characters
+        new_markup ||= " nil "
+      when Expression::INTEGERS_REGEX, Expression::RANGES_REGEX, Expression::FLOATS_REGEX, *LITERALS
+        # Quote variable lookups that match literals after characters are skipped by regex scanning
+        new_markup = "['#{new_markup}']"
+      else
+        new_markup.prepend(" ") if first_match.begin(0) > 0
+        new_markup << ' ' if last_match.end(0) < markup.length
+      end
+
+      new_markup
+    end
+
     def initialize(markup)
       lookups = markup.scan(VariableParser)