[API] Dropdown button contents - TypeScript

[clayplumridge]

For the TypeScript definitions for Dropdown, the title element supports more than just string by the current implementation. Expanding it to JSX.Element would allow for rendering of custom children within the button, including Octicons and other elements without any changes to the existing implementation.

[dmarcey]

I guess it comes down to - should a consumer be able to pass non-string content in the title prop? If so, should we change the name of the prop from title (a breaking change), or just update the typing to be what the component actually supports (`React.ReactNode or JSX.Element like @clayplumridge suggests).

Here's a codesandbox that shows the issue (and also another random typing bug with the icon prop of <TextInput />.

[emplums]

Hey! Looking into this now - I think it would be fine to expand the props of title here to allow a react node or a string, since there are several cases where you might want to pass in an Octicon. I will update the type definition!

Just a heads up - in the next major release I'm planning on refactoring the Dropdown component to allow users to pass a component to override the entire button used as the summary to allow more flexibility since many of our dropdown buttons do not use the primary styles.