Fix width of emoji characters with VS16
VS16 (emoji presentation variation) may change the width of some emoji characters from 1 to 2. With this commit, konsole changes the character width in those cases.
notcurses-info before and after (note the atom emojis in the middle):
The VS16 character cannot be stored in the extended characters, since in some fonts (specifically, noto color emoji) ZWJ sequences with VS16 do not render correctly.
This requires a change in decoding screen to text, as stringWidth() gives incorrect results in this case.