Add the ability to load .cs files from script files (.csx)

[0x53A]

ref dotnet/roslyn#4478

Not being able to reference a normal C# file (.cs) from a script really hampers the productivity of using .NET Interactive workbooks.

I have a CSV file with data, and a Models.cs file which contains - you guessed it - the schema for the data.
What I'd like to do is

Open VS Code, create a new Workbook.

#load ".\models.cs"
var text = File.ReadAllText("data.csv");
var rows = Models.LoadFromText(text);

// display(rows)

The proposed workaround is to compile the c# file into a dll and then #r that.
This works fine - the first time.

But the workbook locks the DLL!
If I need to change something, I need to close VS code, recompile the dll, then restart the workbook.
This sucks.

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Please note that I don't specifically need to use namespaces in a C# script. The linked issue describes why that was not implemented. Instead I'd like to import normal C# (not script) files into a script. It's fine if the script executor compiles the .cs file(s) into a separate assembly and then loads that, as long as it does it transparently in the background.