"Forbid export" feature; and fix bug in makeAddPgTableConditionPlugin

[benjie]

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:

crystal/graphile-build/graphile-utils/src/makeAddPgTableConditionPlugin.ts

Lines 85 to 86 in 6401dea

	conditionFieldSpec.applyPlan = EXPORTABLE(
	(build, conditionGenerator, sql) =>