Allow creating playground from default manifest

[sagikazarmark]

When no manifest file is provided, use the builtin default manifest (using --base)

[sagikazarmark]

@iximiuz what do you think?

[iximiuz]

What are the intended use cases? 🤔

[sagikazarmark]

Replacing the following:

labctl playground manifest flexbox > manifest.yaml
# yq replace name and base
labctl playground create --file manifest.yaml my-playground

with

labctl playground create --base flexbox my-playground

[iximiuz]

Ah, I see. Yep, let's merge it. I'd only try simplifying the ifs there - it's a bit too much of nested logic. Maybe extract the whole manifest reading into a function that can do early returns to avoid nested conditions?

By the way, --base can probably default to flexbox.