Add a project template for source generators

[hez2010]

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

It's inconvenient that whenever I want to author a source generator, I will need to search for the document to see which packages to reference and the necessary msbuild properties and items to include.

It would be great if we can have a project template for source generators.

Describe the solution you'd like

Add a project for source generators so that one can create a source generator by executing:

dotnet new source-generator

Additional context

Add any other context or screenshots about the feature request here.

[baronfel]

Hey @jaredpar - does Roslyn have a tracking issue for this already?

[jaredpar]

No.