Support `label-number` format in pre-release tags

[eduherminio]

One possible improvement is to also support label-number in the pre-releases, since I noticed that multiple packages use it (and I do it myself from time to time, not always on purpose). i.e.,

> dotnet version -f FileParser.csproj --skip-vcs 1.6.1-beta-1
dotnet-version-cli version 2.0.0.0
Bumped FileParser.csproj to version 1.6.1-beta-1

> dotnet version -f FileParser.csproj --skip-vcs prerelease
dotnet-version-cli version 2.0.0.0
ERR Pre-release part invalid. Must be either numeric or `label.number`. Got beta-1

Extending that to labelnumber (beta1, beta2, etc.) would be a nice to have, but I don't know if that would complicate things.

Depends on https://github.com/skarpdev/dotnet-version-cli/issues/33, but let's track them separately so that v2.0.0 can be released without having to decide on this suggestion.