Fix using partial! with digging

When digging, we swap the builder instance, then when encountering `partial!`
on an instance that does not have it implemented, it errors out. This moves the
method to the top level top level props_template. So both instances have it.

Author: jho406

lib/props_template.rb

@@ -64,6 +64,10 @@ def set!(key, options = {}, &block)
       end
     end
 
+    def partial!(**options)
+      @context.render options
+    end
+
     def found_path!
       @found_path.join(".")
     end

lib/props_template/base.rb

@@ -111,10 +111,6 @@ def array!(collection = nil, options = {})
       nil
     end
 
-    def partial!(**options)
-      @context.render options
-    end
-
     # json.id item.id
     # json.value item.value
     #