`@ref` should allow specifying the `from` so single-table polymorphism can apply refs to a single type
Effectively we just need to add an optional `from:` clause to `@ref` and check that when we're adding it.

Reported by @enisdenjo.

If you wanted to take it on, destructure from here:

https://github.com/graphile/crystal/blob/4f2fb57a2f2b89cc83834c0f951dc7682e96a94b/graphile-build/graphile-build-pg/src/plugins/PgRefsPlugin.ts#L135

add it here as `graphqlTypeFrom` or `sourceGraphQLType` or similar:

https://github.com/graphile/crystal/blob/4f2fb57a2f2b89cc83834c0f951dc7682e96a94b/graphile-build/graphile-build-pg/src/plugins/PgRefsPlugin.ts#L156

filter the refs here:

https://github.com/graphile/crystal/blob/4f2fb57a2f2b89cc83834c0f951dc7682e96a94b/graphile-build/graphile-build-pg/src/plugins/PgRelationsPlugin.ts#L754-L766 to ensure they either have no from or they match context.Self.name (IIRC; if not; TypeScript should help)