Playground common

This package contains common code for the playground packages
that doesn't fit in any other package. For example, the
RecommendedPHPVersion is imported virtually everywhere
and including it in any other specific package would result
in a lot of circular dependencies.

Avoid adding new code to this package. @wp-playground/common should remain
as lean as possible. It only exists to avoid circular dependencies. Let's not
use it as a default place to add code that doesn't seem to fit
anywhere else. If there's no good place for your code, perhaps
it needs to be restructured? Or maybe there's a need for a new package?
Let's always consider these questions before adding new code here.