Mark public API exports

Recharts has a number of exported types which are public API and we should be extra careful before making any changes to them. From within the codebase it is not at all obvious which type or component is exported and which one is internal. We need some way to distinguish - easier than searching in all exports and checking if one of them is index.ts.

I imagine we can extend omnidoc tests to enforce that all public exports have some sort of JSDoc tag. This tag will be informational only and will not render anything on the website (which by nature of things only renders public API).