Commands to work with states available for WorkItem types

[jhoneill]

PR Summary

Follow on from #361 Add support for workitem states

• Get-VsteamWorkItemState
• Add-VSTeamWorkItemState
• Remove-VsteamWorkItemState
• Hide-VsteamWorkItemState
• Show-VsteamWorkItemState and associated Format file + tests and help

Each WorkItem-type in each process-template has a set of possible states e.g. "To Do", "In Progress" and "Done". Some are system states which cannot be removed but can be hidden (and shown). Other states are custom/user-added and can be removed (but not hidden). This PR provides tools for listing the states, hiding and showing system ones, and adding and removing user ones.

PR Checklist

• [X] Write Help
• [X] Write Unit Test
• [X] Update CHANGELOG.md

[SebastianSchuetze]

gonna close this PR since it is too old and actually too much changes to review. If reopened then it must be made sure that no PR is directly dependant on the other. But thanks for the effort.