Importmap support

[yidafu]

insert 'importmap' script tag in head.

<script type="importmap">
  {
    "imports": {
      "three": "https://cdn.jsdelivr.net/npm/three@<version>/build/three.module.js",
      "three/addons/": "https://cdn.jsdelivr.net/npm/three@<version>/examples/jsm/"
    }
  }
</script>

then, we can import module like:

import * as THREE from 'three';
import { OrbitControls } from 'three/addons/controls/OrbitControls.js';
import { GLTFLoader } from 'three/addons/loaders/GLTFLoader.js';