Add displayName to FormikContext (jaredpalmer#2962)

Right now the context doesn't have a displayName, and the default name for a context is just "Context" so in an app with a lot of contexts we end up with several items in the devtools call `<Context.Provider>` and `<Context.Consumer>`

![image](https://user-images.githubusercontent.com/709773/102159836-2e4c8180-3ee9-11eb-82b9-b97fc1e5d199.png)

this PR will make the formik ones display as

`<FormikContext.Provider>` and `<FormikContext.Consumer>`

![image](https://user-images.githubusercontent.com/709773/102159868-42907e80-3ee9-11eb-83b2-822ea842464a.png)

This also has the added benefit of making it easier to quickly locate the Formik context when using the component search.

packages/formik/src/FormikContext.tsx

@@ -5,6 +5,8 @@ import invariant from 'tiny-warning';
 export const FormikContext = React.createContext<FormikContextType<any>>(
   undefined as any
 );
+FormikContext.displayName = 'FormikContext';
+
 export const FormikProvider = FormikContext.Provider;
 export const FormikConsumer = FormikContext.Consumer;