NuGet: Add support for .msbuildproj project file extension

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Feature description

Add support for project file extension: _.msbuildproj_

This file extension is often used in combination with _Microsoft.Build.NoTargets_ project types (https://github.com/microsoft/MSBuildSdks/tree/main/src/NoTargets) and can contain package references as well.

See e.g. https://github.com/NuGet/NuGet.Client/blob/dccbd304b11103e08b97abf4cf4bcc1499d9235a/src/NuGet.Core/NuGet.Build.Tasks/NuGet.targets#L369 for usage of this extension in official Microsoft product.

Start by tracing Dependabot's NuGet support for recognized project-file extensions and its related tests. Use the linked NuGet.targets example and Microsoft.Build.NoTargets project as context; done means package references in .msbuildproj files are recognized and covered by tests.

Written by the indexing model from the issue text.