Implement saving/loading of persistent values to configure multiple views

Desired workflow: The user programmatically configures several "views" of their data, each of which corresponds to a different collection of settings (stuff which is enabled, etc). The user wants to quickly switch between these views without clicking a bunch.

How to get there: We could get this fairly easily by leveraging the persistent value caches. Work items would be:

• define a serialization format for persistent values (in-memory for now)
• add a storeConfiguration() function or something, which saves the state of all persistent values to an internal list
• add a loadConfiguration() function which restores persistent values
• buttons in the GUI, etc

This has significant overlap with things we would want to do regardless to enable serialization more broadly.

首先阅读 include/polyscope/persistent_value.h，并追踪 persistent values 的创建和访问方式。在实现配置保存/加载之前，先定义内存中的序列化格式以及所存储状态的范围；只有在核心行为明确并且可测试之后，再考虑 GUI 按钮。