Add custom font picking

[quiprr]

This may be a Mac specific thing? also not sure

[kirb]

For macOS I’ll make it show the font picker immediately; for iOS I’ll show the system/app built-in fonts and provide a button to select a font from the system font picker. Only reason I didn’t just do that already is because the font picker doesn’t list SF Mono, and pretty annoyingly, shows Apple Color Emoji as a monospace font!