Design and develop multithreaded mode support in Visual Studio

Visual Studio hosts the MSBuild main node inside the devenv process and sends most build work to separate processes. In multithreaded mode all project builds would run inside devenv, which risks crashing Visual Studio and using its memory on large builds. That is one problem we already know about, and there are likely more. The goal of this epic is to find all of them, design multithreaded support for Visual Studio, and implement it.

## Steps

- [ ] [#11939](https://github.com/dotnet/msbuild/issues/11939) Prototype and design multithreaded support for Visual Studio, and identify the problems it has to solve [size L]
- [ ] Implement the design [size TBD, depends on the first step]

## Stakeholders

Visual Studio users and the Visual Studio project system teams. Also MSBuild API consumers that host builds inside their own process.

## Definition of done

Visual Studio runs multithreaded builds, the problems found during design are solved.