add a property that's true iff on a dev machine

Something like
```xml
true
```

One way this would be useful is if you are adding a new warning that will affect several repos (eg., an obsoletion) and expect it to trigger errors that will will break their builds, PR validation and codeflow. You could instead conditionalize it to only warn/error if `OnDevMachine` is true. That way devs are immediately broken when it reaches a repo and they quickly push up a PR to fix it. But PR validation and codeflow is not broken. Then a week or two later, you remove the condition.

opening an issue as I don't know where this property would go.

cc @ViktorHofer @lewing

The issue names no file, test, or entry point, so first locate where Arcade defines shared MSBuild properties and review similar conditional properties. Done means identifying an appropriate shared location and adding a property that distinguishes developer builds from continuous-integration and official builds, with validation for the stated conditions.

Written by the indexing model from the issue text.