Change inline snippet identifier from string to variable

Currently, snippet files identified by strings. This
PR makes changes to render to allow for new inline
snippets to use variables as identifiers instead

lib/liquid/tags/render.rb

@@ -27,7 +27,7 @@ module Liquid
   # @liquid_syntax_keyword filename The name of the snippet to render, without the `.liquid` extension.
   class Render < Tag
     FOR = 'for'
-    SYNTAX = /(#{QuotedString}+)(\s+(with|#{FOR})\s+(#{QuotedFragment}+))?(\s+(?:as)\s+(#{VariableSegment}+))?/o
+    SYNTAX = /(#{QuotedString}+|#{VariableSegment}+)(\s+(with|#{FOR})\s+(#{QuotedFragment}+))?(\s+(?:as)\s+(#{VariableSegment}+))?/o
 
     disable_tags "include"
 
@@ -51,12 +51,15 @@ module Liquid
     end
 
     def render_tag(context, output)
-      # The expression should be a String literal, which parses to a String object
       template_name = @template_name_expr
-      raise ::ArgumentError unless template_name.is_a?(String)
 
-      if context[template_name].is_a?(Liquid::SnippetDrop)
-        snippet_drop = context[template_name]
+      # For inline snippets, @template_name_expr is a VariableLookup
+      if template_name.is_a?(VariableLookup)
+
+        snippet_drop = context[template_name.name]
+
+        raise ::ArgumentError unless snippet_drop.is_a?(Liquid::SnippetDrop)
+
         inner_context = context.new_isolated_subcontext
 
         if inherit_context?
@@ -74,6 +77,9 @@ module Liquid
         return output << snippet_drop.body.render(inner_context)
       end
 
+      # Otherwise, the expression should be a String literal, which parses to a String object
+      raise ::ArgumentError unless template_name.is_a?(String)
+
       partial = PartialCache.load(
         template_name,
         context: context,