headless-ui for amis

#### 是否关联于某个问题吗：
更大范围扩大amis已有能力

#### 预期的解决方案：
参考melt-ui，用类似的接口，把组件的元素和行为抽取出来，具体的UI则扔掉或者减少到最小。。。

#### 其他可接受方案：
还没考虑到其他方案。。。

#### 任何附加信息：
这样做貌似可以让amis来做C端的UI了。。。

Start by reviewing amis's existing component architecture and the melt-ui approach referenced in the issue. The proposal does not name files, entry points, tests, or a defined acceptance criterion; work would need an agreed scope for the headless interfaces and a clear definition of done before implementation can begin.

Written by the indexing model from the issue text.