Support pre-release iterations with NuGet Repack

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`).