Migrate from `vstest` to `Microsoft.Testing.Platform`

[Evangelink]

First draft at changing the codebase to be an MTP GH reporter

Closes #41

[Tyrrrz]

Would be nice if we could also update the Demo project to use MTP as well 🙏🏻

[nohwnd]

Hehe, I was actually working on the code to support both at the same time 😁

[nohwnd]

Because I missed this comment, saying that you are okay to migrate:

https://github.com/Tyrrrz/GitHubActionsTestLogger/issues/41#issuecomment-2669057401

fwiw code is here (the MTP part is heavily not done) https://github.com/Tyrrrz/GitHubActionsTestLogger/pull/45 feel free to close.

[Evangelink]

Blocked by https://github.com/microsoft/testfx/issues/5265

I'll address the other comments and do some ccleanup this week,

[martincostello]

This should be unblocked now: https://github.com/microsoft/testfx/issues/5265#issuecomment-3124235442

[Tyrrrz]

@Evangelink do you think we can get this moving again?

[martincostello]

An app where I'm already using .NET 10 RC.1 has been updated to MSTest.Sdk 4.0.0, and now I'm starting to see messages like this:

/home/runner/.nuget/packages/microsoft.testing.platform.msbuild/2.0.0/buildMu
ltiTargeting/Microsoft.Testing.Platform.MSBuild.targets(263,5): error : Testing 
with VSTest target is no longer supported by Microsoft.Testing.Platform on .NET 
10 SDK and later. If you use dotnet test, you should opt-in to the new dotnet 
test experience. For more information, see https://aka.ms/dotnet-test-mtp-error 

It would be unfortunate if I'm forced to drop the GitHub Actions test logging because I have to choose between .NET 10 or VSTest support.

[Tyrrrz]

@martincostello hm, that's harsh, I was hoping for a softer migration process.

I'll take a look and try to restart this PR myself 👍🏻

[Tyrrrz]

I have merged the code into a separate branch so I can continue working on it. Thanks @Evangelink and everyone else for the effort thus far!