Add callback to get missing fonts

[erzu12]

This allows a user to register a callback to be called when a font is missing, as suggested in #178.

For now, I have made the API more C++ style than the other add_font functions, but feedback on this is very welcome.

[sammycage]

Hello @erzu12

Thank you so much for the great idea and for putting time into this! This callback functionality for handling missing fonts is a creative approach, and I can see its potential value.

That said, the current implementation feels a bit complex. It seems to duplicate some of the existing functionality of add_font, which might add unnecessary redundancy. While I appreciate the more C++-style approach here, it introduces an additional layer that could complicate usage for developers accustomed to our existing API.

At this stage, keeping the API streamlined and straightforward is a key priority. Could we consider a simpler way to extend add_font for this purpose instead? Perhaps we could explore a lighter modification that integrates more naturally with the current structure.

Thanks again for your contribution, and looking forward to more of your ideas!