[g-webgl] 支持 WebXR

https://github.com/antvis/g-device-api/issues/38

目前 WebGPU 尚不支持。https://launch.variant3d.com/blog/23-06-state-webxr-on-ios-beyond

Three.js 中 AR 的用法如下：

1. 提供 ARButton：https://github.com/mrdoob/three.js/blob/master/examples/jsm/webxr/ARButton.js
2. Renderer 创建：https://threejs.org/docs/#api/en/renderers/webxr/WebXRManager

Start with the linked g-device-api issue and the referenced Three.js ARButton and WebXRManager documentation. Then inspect the g-webgl renderer entry point and determine how WebXR support should fit there, noting the issue's statement that WebGPU does not yet support it. Done means the requested WebXR AR workflow is supported and documented by appropriate validation.

Written by the indexing model from the issue text.