Enable PSR-4 autoloading by default for new project

[maxime-rainville]

When you create a new project right now, it is not namespace by default and does not include an autoload.psr-4 key in its composer file.

If we're going to nudge people into pre-adopting PSR-4 before it becomes mandatory in SilverStripe 5, making it the default config for new SilverStripe 4 projects could be a good first step.

silverstripe/recipe-plugin might need some work as well for this to work.