Add a project template for source generators

### 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:

```bash
dotnet new source-generator
```

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