Add new dark/light mode compatible logo

Summary:
I'm looking at updating the IGListKit logo with a more brand-appropriate variant, that also works more nicely in both the light and dark mode themes on GitHub.

This diff only adds the logo to the codebase, which will make it easier for me to add and style it in the README in a subsequent diff once it's landed.

This logo is by no means set in stone, so if anyone else has any feedback for it, or would like to propose anything else, I'm totally open. Thanks!

Differential Revision: D56996027

fbshipit-source-id: c2f7170a9aca39f5c2ccbcd897bbe1801624c4c1