Add `/.dotnet` as a supported folder, like `/.config`

### Is your feature request related to a problem? Please describe.

As `.config` is quite generic, could commands such as `dotnet tool` etc also check a `.dotnet` folder?

This enables other files related purely to .NET development to also be stored there. For example, currently we have shared `Directory.Build.props` (and related files) between solutions in a mono repo. They're stored in a folder in the root of the repo, with each solution having it's own `D.B.p` that imports them.

A common/ known folder would be easier/ more obvious to manage.

### Describe the solution you'd like

Add `.dotnet` a known folder for .NET SDK tooling. Even support loading/ looking for `global.json` files there.

### Additional context

Adds clarify, and helps to keep larger repos more organised - e.g. when there's more than just .NET technologies at play, the root of a repo can become quite busy.