fix typescript type assertions with use_import_type_syntax=true

[PatrykWalach]

Overview

Fix for an issue where useImportTypeSyntax: true generates runtime errors in typescript when relay resolvers are used.

The changes are a bit ugly but as per comment the assertions are a short term solution

Tests

I'm guessing it's not possible to toggle feature flags per fixture. I've manually tested this against my project:

• works for live resolvers
• works for normal resolvers

Closes #4818

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