[Feat]: cull_idle_timeout mechanism requirement

## What is your suggestion?
Maybe cull_idle_timeout mechanism should be provided. If there hasn't been an active connection to code-server in X minutes, we should exit with status 0. It has been mentioned in #1636 @nhooyr

## Why do you want this feature?
I am working on develop online VSCode support by code-server. To enhance the resource utilization rate, I require cull-idle-timeout mechanism. If users do not use for a period of time, I hope the container killed. According to the doc, code-server provides the heartbeat mechanism. If I keep the WebPage on but do nothing, the time of heartbeat will be updated every minute. This is not what I want.

## Others
I have seen that such feature is referred like issue #1636. However, I failed to launch with --idle-timeout args. I want to confirm if such function is supported right now? If not, is there any methods to support my requirement? I'll appreciate it if someone could help.

Start by checking whether the existing --idle-timeout argument is implemented and how the heartbeat represents activity; issue #1636 is the related discussion to read. Define how an active connection differs from an idle browser page, then verify that the server exits with status 0 after the timeout and supports the container use case.

Written by the indexing model from the issue text.