Fix conditionals to use [[ and (( rather than [

This fixes a small issue in src/widgets.zsh which makes it so if you
alias [ to g[ (as is done in prezto if the gnu-utility module is loaded)
autosuggestions would fail.

The documentation for GNU test mentions that -o and -a should be avoided
if possible because it's not very clear. Also, with zsh and [[ -o
actually tests if an option is set, which makes this option even more
confusing.

src/start.zsh

@@ -15,7 +15,7 @@ _zsh_autosuggest_start() {
 	# to the widget list variables to take effect on the next precmd.
 	add-zsh-hook precmd _zsh_autosuggest_bind_widgets
 
-	if [ -n "${ZSH_AUTOSUGGEST_USE_ASYNC+x}" ]; then
+	if [[ -n "${ZSH_AUTOSUGGEST_USE_ASYNC+x}" ]]; then
 		_zsh_autosuggest_async_start
 	fi
 }