[FR] support for overline text formatting

[jeffgazso]

Prawn currently lacks support for overline formatting of text. I realize this is niche, but Morse code prosigns use overline formatting as a typographic convention to indicate multiple characters sent back-to-back without letter spacing. Probably the most widely recognized Morse Code prosing is SOS, indicating the sender is in distress.

Documents containing Morse Code prosigns are my use case for this feature, it looks like it's also used in mathematics and for some scribal abbreviations as well.

[bai-yi-bai]

@jeffgazso I asked this question previously and got response about this #1360
I solved this in AsciiDoctor-PDF by manually generating a font with overlines using FontForge. Unfortunately, I did not have the time to invest in learning how to script FontForge to generate fonts.