vi-mode: Shorten names for VI modes on the status bar

Simplify the string representation of the VIM modes to help save space in the crowded status bar.

"VI: INSERT MODE" -> "VI: INSERT"
"VI: NORMAL MODE" -> "VI: NORMAL"

The word "MODE" is already implicit and absent from the other modes ('REPLACE', 'VISUAL', 'VISUAL LINE'). It also makes the representation similar to other editors

BEFORE	AFTER