Validate `graphql-config` using `vscode-graphql` and the available schema json

This is just a few lines of code!

https://raw.githubusercontent.com/kamilkisiela/graphql-config/master/config-schema.json
https://code.visualstudio.com/Docs/languages/json#_mapping-a-schema-in-an-extension

I feel like if I write anything more about it, I'll officially have had spent more time planning this than implementing it as usual 😆