Add `init` command to more easily set up a new project

[xt0rted]

This should do a couple things:

1. If a global.json doesn't exist in the executing folder create one
2. Add a scripts section with a simple echo "hello world" type script

It could also accept a --template <template> option which would add pre-configured scripts. The default one could be something like this:

{
  "scripts": {
    // Debug mode scripts
    "build": "dotnet build",
    "test": "dotnet test",
    "pack": "dotnet pack --output ./artifacts",
    "ci": "dotnet r build && dotnet r test && dotnet r pack",

    // Release mode scripts
    "build:release": "dotnet build --configuration Release",
    "test:release": "dotnet test --configuration Release --no-build",
    "pack:release": "dotnet pack --configuration Release --no-build --output ./artifacts",
    "ci:release": "dotnet r build:release && dotnet r test:release && dotnet r pack:release",
  }
}

Should this be init and only available if no global.json or scripts section are found, or should it be a flag --init and always visible?