window.beforeclose prompt should not alert if running in app mode in chrome

[weikinhuang]

• code-server version: v1.939-vsc1.33.1
• OS Version: linux server, osx client

Description

When running code-server in chrome app mode (ex sshcode) the window close prompt should not show up when doing ctrl+w, since running in app mode, it's able to overload the ctrl/cmd+w shortcut to actually close editor tabs

Steps to Reproduce

1. Start code-server
2. Open app with

/Applications/Google\ Chrome.app/Contents/MacOS/Google\ Chrome --app=https://server-ip:8443

1. Open several editor tabs
2. cmd+w to close 1 tab

[code-asher]

This was removed in v2. 🎉