Add feature detection

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

Based on https://github.com/mafredri/zsh-async/blob/a4b2f81c966a00eeb38876d505d97423cc8addfd/async.zsh#L395-L401

src/features.zsh

@@ -0,0 +1,19 @@
+
+#--------------------------------------------------------------------#
+# Feature Detection                                                  #
+#--------------------------------------------------------------------#
+
+_zsh_autosuggest_feature_detect() {
+	typeset -g _ZSH_AUTOSUGGEST_ZPTY_RETURNS_FD
+	typeset -h REPLY
+
+	zpty $ZSH_AUTOSUGGEST_PTY_NAME :
+
+	if (( REPLY )); then
+		_ZSH_AUTOSUGGEST_ZPTY_RETURNS_FD=1
+	else
+		_ZSH_AUTOSUGGEST_ZPTY_RETURNS_FD=0
+	fi
+
+	zpty -d $ZSH_AUTOSUGGEST_PTY_NAME
+}

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
 }