Refactor async mode to no longer use zpty

See technique used in `fast-syntax-highlighting`:
- ca2e18bbc9
- 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

README.md

@@ -67,7 +67,7 @@ This can be useful when pasting large amount of text in the terminal, to avoid t
 
 ### Enable Asynchronous Mode
 
-As of `v0.4.0`, suggestions can be fetched asynchronously using the `zsh/zpty` module. To enable this behavior, set the `ZSH_AUTOSUGGEST_USE_ASYNC` variable (it can be set to anything).
+As of `v0.4.0`, suggestions can be fetched asynchronously. To enable this behavior, set the `ZSH_AUTOSUGGEST_USE_ASYNC` variable (it can be set to anything).
 
 
 ### Key Bindings