Add customizable default static port

[cat-pat]

Adds a configurable default static port instead of having it be random on each launch, useful for having a bookmark you can quickly refer to when in use

Default is set to a random but memorable 32423 which can be changed to any other port by the user using the markdown.preview.port config options

[El-Wumbus]

How would the port be changed if the aurelius::Server is bound to the default port prior to receiving the updated configuration? At first, settings is set to Default::default(), and is only updated later by the LSP client. The server would need to be restarted after the configuration changed, which I don't see happening here.

I've implemented a fix by restarting the server upon a port change:

// fn handle_notification(...) {
//...

info!("changed configuration: {:?}", settings);

let old_auto_setting = self.settings.auto;
let restart_server = self.settings.port != settings.port;                  

self.settings = settings;

if restart_server {
    match aurelius::Server::bind(format!("localhost:{}", self.settings.port)) {
        Ok(server) => {
            // NOTE: `Drop`ing `aurelius::Server` is enough to close everything.
            *self.markdown_server.lock().unwrap() = server;   
            info!("Restarted markdown server on localhost:{}", self.settings.port);
        }
        Err(e) => {
            error!("Failed to restart markdown server on localhost:{}: {}", self.settings.port, e);
        }
    }
}

// ...