Update how Schema::Field yields definition_block

Previously this used `instance_exec` which sets `self` to the field
instance.

This changes to `yield` which still yields the field instance
(supporting the same functionality) but doesn't change `self`.

lib/graphql/schema/field.rb

@@ -186,7 +186,7 @@ def initialize(type: nil, name: nil, owner: nil, null: nil, field: nil, function
 
         if definition_block
           if definition_block.arity == 1
-            instance_exec(self, &definition_block)
+            yield self
           else
             instance_eval(&definition_block)
           end