Closes https://github.com/microsoft/vscode/issues/168492
This implements @aeschli's 'server server' concept in a new
`code serve-web` command.
Command line args are similar to the standalone web server. The first
time a user hits that page, the latest version of the VS Code web server
will be downloaded and run. Thanks to Martin's previous PRs, all
resources the page requests are prefixed with `/<quality-<commit>`.
The latest release version is cached, but when the page is loaded again
and there's a new release, a the new server version will be downloaded
and started up.
Behind the scenes the servers all listen on named pipes/sockets and the
CLI acts as a proxy server to those sockets. Servers without connections
for an hour will be shut down automatically.
* cli: ensure ordering of rpc server messages
Sending lots of messages to a stream would block them around the async
tokio mutex, which is "fair" so doesn't preserve ordering. Instead, use
the write_loop approach I introduced to the server_multiplexer for the
same reason some time ago.
* fix clippy
Other connected clients will now print:
```
Connected to an existing tunnel process running on this machine. You can press:
- Ctrl+C to detach
- "x" to stop the tunnel and exit
- "r" to restart the tunnel
```
These are then sent to the server to have that take effect. This is
mostly some refactors in the singleton_server to make the lifecycle work.
