Add support for associating a notebook with a project (`#r project`)

[jonsequitur]

The question comes up occasionally about whether a project can be referenced from a notebook. The use cases vary, so a discussion would be useful to understand the kinds of things people might want to use such a mechanism for.

Example scenarios:

• Initializing a notebook's NuGet dependencies
• Defining methods and classes that can be used from within the notebook

Some questions:

• Is it desirable to have an "automatic" association of some kind so that code from the project is invoked automatically based on some convention (e.g. the presence of a project with the same name in the notebook's folder)?
• What's the workflow for seeing changes in the associated project reflected back in the notebook?
• What kinds of projects should be supported?

[onionhammer]

Personally I see this feature being useful for the following:

1. I have a large domain project (csproj) that isnt a nuget package, but I want to reference it to generate stats, charts, reports etc from the domain (which accesses a database).
2. Building code that can be shared by multiple notebooks.

It would be great if an active notebook reloaded references when the project was rebuilt, but I think the user could just manually re-run the referencing cell as well

[thelazydogsback]

Also was looking for this so I could experiment with some of our internal libs along side of projects and provide "live documentation" for them.

It would also be great to see these notebooks integrated with the UnitTest runner, so I could interactively write testing code and turn them into unit tests by having cells do Asserts/Expects.

[jonsequitur]

Related: dotnet/roslyn#51480

[jdebache]

Agreeing with previous comments : this feature would be a great way of creating interactive demos for library code.