[Feature idea] Add ability to group project for build/test

Hi it is possible to add ability to specify projects groups based on trait|category|group?
This could replace slnf, so instead of having several slnf files in repository we could have only one slnx file where could be projects annotated with props. i.e Category|Group...

Example

<Solution>
  <Configurations>
    <BuildType Name="Debug" />
    <BuildType Name="Release" />
  </Configurations>
  <Folder Name="/Solution Items/">
    <File Path=".editorconfig" />
    <File Path="README.md" />
    <File Path="NuGet.Config" />
    <File Path="Directory.Build.props" />
  </Folder>
  <Folder Name="/test/">
    <Project Path="test\FunctionalTests\FunctionalTests.csproj" />
    <Project Path="test\UnitTests\UnitTests.csproj" Category="UnitTests"/>
    <Project Path="test\UnitTests1\UnitTests1.csproj" Category="UnitTests"/>
  </Folder>
  <Folder Name="/src/">
    <Project Path="src\My.API\My.API.csproj" Category="Group1"/>
    <Project Path="src\My.API1\My.API1.csproj" Category="Group2"/>
    <Project Path="src\My.API2\My.API2.csproj" Category="Group1"/>
    <Project Path="src\My.API3\My.API3.csproj" Category="Group1"/>
    <Project Path="src\My.API4\My.API4.csproj" Category="Group2"/>
    <Project Path="src\My.API5\My.API5.csproj" Category="Group1"/>
  </Folder>
</Solution>

Usage

1. dotnet test --filter Category=UnitTests
   this should build&test test\UnitTests\UnitTests.csproj and test\UnitTests1\UnitTests1.csproj
2. dotnet build --filter Category=Group2
   this should build src\My.API1\My.API1.csproj and src\My.API4\My.API4.csproj

Start by reviewing how this repository represents and persists .sln and .slnx solutions, then trace how the dotnet build and dotnet test commands select projects. Define the grouping syntax and filtering behavior needed for the UnitTests and Group2 examples, and consider compatibility with existing slnf files. Done means the shown commands select exactly the expected projects.