Set-VSTeamVariableGroupVariable cmdlet, tests, docs

[sevaa]

PR Summary

Addresses #423

PR Checklist

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

[sevaa]

Addressed most. Please take another look.

[sevaa]

What are my next steps, please?

The group search by name is broken (obscure exception if not found), the double JSON convert can be avoided but the review doesn't say so, just asks why.

[sevaa]

Is this PR a lost cause? If so, please let me know. As outlined in #423, I'll go ahead and publish as a separate PowerShell module.

[sevaa]

Any update please? Any maintainers present? I think I've made all the changes needed - but the PR still says "Changes requested". @SebastianSchuetze @DarqueWarrior

[sevaa]

@SebastianSchuetze What should be my actions now?

[SebastianSchuetze]

Thanks for the help! Gonna merge it.