If prerelease metadata doesn't contain numbers, the Version task should append the prerelease number

[splatteredbits]

If a prerelease label doesn't have a number in it, the Version task should append the prerelease number.

Or, use a $(WHISKEY_NEXT_PRERELEASE_VERSION) variable? Or support both?

The semver spec allows label-less prerelease versions. Make sure those are supported, too.

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