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?

Start by locating the standalone PropTypes implementation and the existing primitive validators; the issue does not name specific files or tests. Confirm the expected behavior for Symbol values and non-Symbol values, then verify that the new PropTypes.symbol validator behaves consistently with the existing validators.

Written by the indexing model from the issue text.