[Feature Request] Add Support for MSBuild Conditionals and Properties.

It would be very handy to be able to use MSBuild Conditional statements and MSBuild Properties within the slnx file.
This would allow for conditional inclusion or exclusion of projects and also to allow paths to dependent projects be configurable.

<Solution>
  <Configurations>
    <Platform Name="Any CPU" />
  </Configurations>
  <Project Path="C\C.csproj" Condition="Exists('C\C.csproj')" />
  <Project Path="B\B.csproj" />
  <Project Path="A\A.csproj">
    <BuildDependency Project="B\B.csproj" />
    <BuildDependency Project="C\C.csproj" Condition="Exists('C\C.csproj')"/>
  </Project>
</Solution>

or

<Solution>
  <Configurations>
    <Platform Name="Any CPU" />
  </Configurations>
  <Project Path="$(PathToC)\C.csproj" Condition="Exists('$(PathToC)\C.csproj')" />
  <Project Path="B\B.csproj" />
  <Project Path="A\A.csproj">
    <BuildDependency Project="B\B.csproj" />
    <BuildDependency Project="$(PathToC)\C.csproj" Condition="Exists('$(PathToC)\C.csproj')"/>
  </Project>
</Solution>

Start by reading the repository's .slnx solution models and parsing or serialization entry points, then compare how the XML examples represent projects and build dependencies. The work is done when MSBuild Conditions and Properties can control project inclusion, dependency inclusion, and configurable paths as shown in the issue, with behavior verified by the repository's tests.