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

spec/integrations/client_zpty_spec.rb

@@ -1,10 +0,0 @@
-describe 'a running zpty command' do
-  let(:before_sourcing) { -> { session.run_command('zmodload zsh/zpty && zpty -b kitty cat') } }
-
-  it 'is not affected by running zsh-autosuggestions' do
-    sleep 1 # Give a little time for precmd hooks to run
-    session.run_command('zpty -t kitty; echo $?')
-
-    wait_for { session.content }.to end_with("\n0")
-  end
-end