allow toString directive (check isNode on merge if inherited from prototype)

[lesleydreyer]

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Description

I was trying to make a toString directive and makeExecutableSchema was failing.

It wasn't working because it creates an object (mergedResultMap in function mergeGraphQLNodes) and objects have an inherited toString method. So it was finding that toString exists in the map because it was the inherited object function, not a directive. (https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/hasOwnProperty#direct_vs._inherited_properties)

So I thought maybe when merging the directive it could check if it exists and if it's an inherited property and not a node, then set to undefined so it can proceed to create the directive node instead of the inherited object function.

Related #5967

Type of change

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• New feature (non-breaking change which adds functionality)

Screenshots/Sandbox (if appropriate/relevant):

Codesandbox to show the error https://codesandbox.io/p/devbox/brave-visvesvaraya-49xx8m

How Has This Been Tested?

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• Unit test
• Patched into my project

Test Environment:

• OS:
• @graphql-tools/...:
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Further comments

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[ardatan]

Thanks for the PR!