Publishing dev builds to myget.org

[OsirisTerje]

I've created a build action to publish to myget.
I set the prefix for dev builds to alpha to get the versioning to behave well. The suffix we had earlier using dev would be above a possible beta.
Further, I used the MinVer client to get a reasonable numbering scheme.

This build triggers on main, master, release, and v3. I added the main now, because we have a task to move the master branch to main asap, I believe just after we have released 4.1 should be good.

We can then use a copy of this builddef for publishing to nuget.org, but just set it without any triggers, so we do it manually when we feel ready.

There are now packages built from this in https://www.myget.org/feed/nunit/package/nuget/NUnit and https://www.myget.org/feed/nunit/package/nuget/NUnitLite

[stevenaw]

Thanks @OsirisTerje
Looks good, no concerns on my end from a phone review.
I'll defer to you to merge in case you'd prefer a review from someone in a position to run it