feature: add doNotDisturb mode

[Tristano8]

This is to prevent the Output window from being focused on every runOnSave command that throws a non-zero exit code (such as when there's a formatting error in a file). There are other tools that may catch this and it's convenient to keep the Terminal window focused at all times (i believe this was the original intention of "backend" mode)

Tested by creating the following command in an Extension Development Environment with the following settings.json:

{
  "runOnSave.commands": [
    {
        "globMatch": "**/*.js",
        "command": "exit 2",
        "runIn": "backend",
        "doNotDisturb": true
    }
  ]
}

[pucelle]

Published v1.9.0, include this merge, thanks.