Support for PropTypes.symbol

I am used to the react `PropTypes` and I used them for things other than components.

I am using `proptypes` but now I need to validate for `Symbol`. This is not in the ~14 code base that is your current implementation.

Do you have any guidance?

首先定位独立的 PropTypes 实现和现有的基本类型值验证器；该 issue 未指定具体的文件或测试。确认 Symbol 值和非 Symbol 值的预期行为，然后验证新的 PropTypes.symbol 验证器是否与现有验证器的行为一致。

由索引模型根据 Issue 内容生成。