Add Liquid::ParseContext#parse_expression for liquid-c node disabling (#1333)

We would like to be able to disable liquid-c VM rendering at runtime,
but right now expression parsing is done using Expression.parse, which
isn't aware of the parse context.  That prevents us from conditionally
compiling to VM code based on a parse option.

lib/liquid/condition.rb

@@ -45,8 +45,8 @@ module Liquid
       @@operators
     end
 
-    def self.parse_expression(markup)
-      @@method_literals[markup] || Expression.parse(markup)
+    def self.parse_expression(parse_context, markup)
+      @@method_literals[markup] || parse_context.parse_expression(markup)
     end
 
     attr_reader :attachment, :child_condition