Support `--dry-run`

When running `dotnet new ...`, a `--dry-run` flag is supported to list files that would be created without actually creating them. In Angular CLI schematics, or Nx generators the same flag is supported to list files that would be created/updated/deleted.

This flag allows developers using the tooling to preview changes and make sure they match their anticipated output.

Additionally, I'd like to add support to aspnet-codegenerator inside nx-dotnet, an Nx plugin for .NET. Given this, people would expect the --dry-run output to work for these scaffolding recipes (see https://github.com/nx-dotnet/nx-dotnet/issues/18)

A `--dry-run` flag is added to the tool that lists modified or created files.