Use uv_set_vterm_state() to override libuv's guess.
See https://github.com/libuv/libuv/pull/1873/ for discussion.
This commit uses a terminal-detection approach based on
GetProcessImageFileNameW(...), which will be reverted in the following
commit. The approach was intended to handle the case of running in
winpty (:terminal), but we will add $NVIM env var for that.
Also add some support for ConEmu, cygwin.
