Directory.Build.targets not supported about versionize

[cabauman]

I've never seen a project put version in Directory.Build.targets before (I normally see it in Directory.Build.props), but if that's an actual use case, I suppose it would be easy to add (basically the same as this commit to support *.props files).

[WeihanLi]

It's simple to get the XML node inner text, while sometimes we may want to reference other variables, currently, we get the inner text only.

[cabauman]

We'd be happy to add the targets file as a supported extension. But in your example snippet it still wouldn't work because Versionize only works with explicit version numbers (e.g. 1.2.3); not evaluated ones e.g. $(VersionPrefix). This is because Versionize is responsible for writing the updated version back to the file where it read from.