Implement new axzsh_is_widechar_terminal() function

Implement new axzsh_is_widechar_terminal() function

This function tries to detect if the current terminal can display "wide
characters" or not.

Idea:
<https://unix.stackexchange.com/questions/184345/detect-how-much-of-unicode-my-terminal-supports-even-through-screen>

At the moment, this function is used by the "axemoji" theme and the
default "right side error prompt" to not use wide characters that would
destroy the layout of the terminal used.