Add support for Directory.Build.props

[samsmithnz]

This allows you to set variables at a top level, that are used in projects.

e.g.
Directory.Build.props

    <NetCoreTargetFramework>net6.0</NetCoreTargetFramework>
    <NetStandardTargetFramework>netstandard2.0</NetStandardTargetFramework>
    <NetFrameworkTargetFramework>net472</NetFrameworkTargetFramework>

a project file in a sub-directory

    <TargetFramework>$(NetCoreTargetFramework)</TargetFramework>

More details about how Directory.Build.props works in docs: https://docs.microsoft.com/en-us/visualstudio/msbuild/customize-your-build?view=vs-2022

To solve:

1. When a variable is detected, lookup the value in a parent directory via Directory.Build.props
2. Use the variable value as the counter

or

1. detect Directory.Build.props file.
2. Save variables/state of file
3. When a variable is found in a project file, in a sub-dir, attempt to substitute in the correct value