[ENH]: Add multiple span selector

還沒有人認領這個 Issue。

### Problem

`matplotlib.widgets.SpanSelector` is a useful widget, however only supports one span to be created because of how the class functionality tracks movement and positioning on a screen.

### Proposed solution

Would there be interest in an `NSpanSelector`, which would generalize SpanSelector to N many spans controlled by a single object?
I've written a demo for `NSpanSelector` which is subclass of `SpanSelector`, that corrects the handling functionality for multiple spans. I haven't contributed to many open source projects before. I'm not sure if something like this would even be wanted by others - I had a particular use case where two selectors were needed simultaneously. But since I've written it I'm happy to contribute it!

Here's a snapshot of my example implementation.

首先檢視 matplotlib.widgets.SpanSelector，以及 issue 中描述的 NSpanSelector 示範提案。在實作之前，釐清是否需要多 span API、其互動模型，以及哪些測試或範例可以定義完成標準。

由索引模型根據 Issue 內容生成。