Support pre-release iterations with NuGet Repack

[mmitche]

The NuGet repack task does support the pre-release iteration. It supports a single pre-release label, dropping the iteration when creating stable preview packages.

This should be supported. Here are the patterns:

• If semver1, then no special handling. Arcade's pattern is to append iteration directly to the pre-release label
• If semver2, then if the non-stable version number has > 2 parts, take the first two (e.g. beta.1). Otherwise take only the first (beta).