Add feature detection

Checks whether `zpty` gives a file descriptor, which was not the case in
older versions of zsh.

Based on a4b2f81c96/async.zsh (L395-L401)

src/start.zsh

@@ -7,6 +7,7 @@
 _zsh_autosuggest_start() {
 	add-zsh-hook -d precmd _zsh_autosuggest_start
 
+	_zsh_autosuggest_feature_detect
 	_zsh_autosuggest_check_deprecated_config
 	_zsh_autosuggest_bind_widgets
 }