add useTheme to ThemingType

[hawkrives]

Summary

Currently, Flow will yell loudly if you try to use useTheme from the result of createTheming; with this change, it's happy.

Test plan

Before, if you did

const {ThemeProvider, withTheme, useTheme} = createTheming(theme)

in a Flow project, you would get "unknown item useTheme".

With this change, Flow is silent, and properly assumes the return of a T-shaped object from useTheme.