Custom Props support in Directory.Build.props/targets

[dansiegel]

Description

Currently the Uno.Sdk provides a schema that includes a number of custom Properties as well as item types that are used by Uno. This exposes an issue however where users get a squiggle on those custom properties when they are in something like a Directory.Build.props file

[just-ero]

I would like to expand on this: I have a ./props/ directory in my repo. There are multiple *.props files in that directory. In the root of the repo I then have a Directory.Build.props file with these contents:

<Project>
  <Import Project="props/*.props" />
</Project>

I would like this case to work as well.