PR view comments should have a maximum width, with the code view using a horizontal scrollbar

This confused be briefly:

![Recording 2023-08-05 at 07 10 26](https://github.com/microsoft/vscode-pull-request-github/assets/2193314/15164167-9298-4e4c-87ba-49e818cec7e6)

It was caused by aa inline base64 file:

![image](https://github.com/microsoft/vscode-pull-request-github/assets/2193314/cd485b22-c797-4b86-9247-83b18fe1d2a6)
