Feat: Better support for union types in records #4

At the moment, it's not really very feasible to create a new record containing a union type in GraphQL. This is because the mandatory $type field for type differentiation in unions collides with the GraphQL syntax for variables and so cannot be represented. This is a pretty notable issue, given that even core Bluesky types like app.bsky.feed.post use union types liberally. Is there a good workaround for this in GraphQL? It doesn't seem to like escaping field names in quotes, but I've been having some other issues getting GraphiQL to play nice so I'm not sure how illegal it is.