feat: exports "PRIMARY_KEY" and "ENTITY_TYPE" symbols (#191)

src/index.ts

@@ -1,7 +1,10 @@
 export { factory } from './factory'
 export { primaryKey } from './primaryKey'
-export { nullable } from './nullable';
+export { nullable } from './nullable'
 export { oneOf } from './relations/oneOf'
 export { manyOf } from './relations/manyOf'
 export { drop } from './db/drop'
 export { identity } from './utils/identity'
+
+/* Types */
+export { PRIMARY_KEY, ENTITY_TYPE } from './glossary'