Add Emoji support

[jhogervorst]

I'm opening this issue since I am using dompdf in a project, and currently it doesn't support showing emoji characters (even when loading a compatible font, like Noto Emoji). There's already an issue requesting emoji support in the dompdf repo (dompdf/dompdf#1698), but according to the replies there, emojis should first be supported in php-font-lib.

[AndyGaskell]

This seems an ok short term workaround...
https://www.beaubus.com/blog/add_emoji_support_to_dompdf.html

[bsweeney]

I believe this would be fixed by adding support for Unicode planes beyond the Basic Multilingual Plane. #140 is at the very least a good start. I haven't reviewed that PR and the relevant specs yet so I can't say if any additional work would be required.