feat: support shape type extension

[Aarebecca]

Now G6 support shape type extension, you can register or get by as follows:

import { register, getExtension, ExtensionCategory } from '@antv/g6';
import { Circle } from '@antv/g';

register(ExtensionCategory.Shape, 'circle', Circle);

const CircleCtor = getExtension(ExtensionCategory.Shape, 'circle');

The upsert method also support specific registered shape type instead of class.

import { Circle } from '@antv/g';

this.upsert('shape-key', Circle, { r: 10 }, container); // General

this.upsert('shape-key', 'circle', { r: 10 }, container); // ✅ Both Ok