Add support for interface on interfaces to transformSchema

[IvanGoncharov]

This change fixes transformSchema, which previously left
GraphQLInterface::interfaces field as-is after transforming types.
This meant that interfaces types used implemented by other interfaces
would appear duplicated in the schema.

TODO:

• Update CHANGELOG.md* with your change (include reference to issue & this PR)
• Make sure all of the significant new logic is covered by tests
• Rebase your changes on master so that they can be merged easily
• Make sure all tests and linter rules pass

[hwillson]

Thanks for this @IvanGoncharov. The NodeJS 14 Windows test is failing because of this:

We're checking to make sure the NODE_VERSION version of 14.16.1 is installed, but after asking to have that version installed, the version that is actually being installed is 14.17.5. So the Assert Node.js version check fails.

It's very strange that nvm use 14.16.1 is picking 14.17.5, but as you mentioned this error isn't caused by your changes. We can look at that separately.

[hwillson]

This looks great @IvanGoncharov - thanks!