Refactor async mode to no longer use zpty

See technique used in `fast-syntax-highlighting`:
- https://github.com/zdharma/fast-syntax-highlighting/commit/ca2e18bbc9e27b9264206c257d2ab68838162c70
- http://www.zsh.org/mla/users/2018/msg00424.html

Also see http://www.zsh.org/mla/users/2018/msg00432.html

In async response handler:
- We only want to read data in case of POLLIN or POLLHUP. Not POLLNVAL
  or select error.
- We always want to remove the handler, so it doesn't get called in an
  infinite loop when error is nval or err.

There is an upstream bug that prevents ctrl-c from resetting the prompt
immediately after a suggestion has been fetched asynchronously. A patch
has been submitted, but a workaround for now is to add `command true`
after the exec.

See https://github.com/zsh-users/zsh-autosuggestions/issues/364

src/widgets.zsh

@@ -95,7 +95,7 @@ _zsh_autosuggest_modify() {
 
 # Fetch a new suggestion based on what's currently in the buffer
 _zsh_autosuggest_fetch() {
-	if zpty -t "$ZSH_AUTOSUGGEST_ASYNC_PTY_NAME" &>/dev/null; then
+	if [[ -n "${ZSH_AUTOSUGGEST_USE_ASYNC+x}" ]]; then
 		_zsh_autosuggest_async_request "$BUFFER"
 	else
 		local suggestion