Does this issue occur when all extensions are disabled?: Yes/No

• VS Code Version: 1.63.0 (the code seems to be the same at main)
• OS Version: Mac 12.2.1 / web

Steps to Reproduce:

1. Open vscode in a browser tab
2. Switch to the PWA
3. Open a new window
4. Observe that the new window opens in a new browser tab, rather than a new standalone window

This is because whether it opens in a new standalone window is determined by the constant isStandalone.
However, this property isn't constant, resulting in unexpected behaviour if it changes.

It would be better to instead make this a method. We can avoid having to poll each time by adding a watcher to this method.

I'll follow up with a PR.