MaterialCreator is never exported

[geyang]

• three version: 170
• @types/three version: 270
• three-stdlib version: 2.35.2

Problem description:

Can not import MaterialCreator.

Relevant code:

import { MaterialCreator } from 'three-stdlib'

Suggested solution:

add MaterialCreator to export: https://github.com/pmndrs/three-stdlib/blob/main/src/loaders/MTLLoader.js#L450