"Forbid export" feature; and fix bug in makeAddPgTableConditionPlugin
`build` should never be exported; it's huge and contains loads of things that are likely not exportable without sufficient work. However, we only meet this issue when someone tries to export the schema - it would be great to flag this issue earlier.

I suggest that we forbid exporting of particular objects (e.g. by adding some custom property such as `$$export: false`) and then have `EXPORTABLE` itself throw if it sees these values. This will help catch these problems at source, for example this one:

https://github.com/graphile/crystal/blob/6401dea84142a9e4491ea1bde34cec8a453f29e6/graphile-build/graphile-utils/src/makeAddPgTableConditionPlugin.ts#L85-L86