Add sleep duration that configurable via ENV

This sleep duration will be used to give a leeway time for some
terminals that haven't loaded all the necessary scripts when the async
job is being dispatched. In some cases, it will make the async job not
working properly or even not running at all.

We set the default sleep duration as `0` to keep backward compatibility
with the previous version and it can be changed through ENV.

async.zsh

@@ -11,6 +11,9 @@
 typeset -g ASYNC_VERSION=1.8.6
 # Produce debug output from zsh-async when set to 1.
 typeset -g ASYNC_DEBUG=${ASYNC_DEBUG:-0}
+# Add sleep duration to give a leeway time for some terminal that haven't load
+# all the necessary script when the async job is being dispatch.
+typeset -g ASYNC_SLEEP_DURATION=${ASYNC_SLEEP_DURATION=0}
 
 # Execute commands that can manipulate the environment inside the async worker. Return output via callback.
 _async_eval() {
@@ -28,6 +31,8 @@ _async_job() {
 	# Disable xtrace as it would mangle the output.
 	setopt localoptions noxtrace
 
+	sleep $ASYNC_SLEEP_DURATION
+
 	# Store start time for job.
 	float -F duration=$EPOCHREALTIME