TS: add missing export on 'BuildSchemaOptions' type (#2457)

Author: IvanGoncharov

.eslintrc.yml

@@ -433,7 +433,6 @@ overrides:
       - plugin:import/typescript
     rules:
       flowtype/require-valid-file-annotation: off
-      import/named: off
 
       ##########################################################################
       # `@typescript-eslint/eslint-plugin` rule list based on `v2.17.x`

src/utilities/buildASTSchema.d.ts

@@ -3,7 +3,7 @@ import { Source } from '../language/source';
 import { GraphQLSchema, GraphQLSchemaValidationOptions } from '../type/schema';
 import { ParseOptions } from '../language/parser';
 
-interface BuildSchemaOptions extends GraphQLSchemaValidationOptions {
+export interface BuildSchemaOptions extends GraphQLSchemaValidationOptions {
   /**
    * Descriptions are defined as preceding string literals, however an older
    * experimental version of the SDL supported preceding comments as