new DSL form container :nested => <inner>

Note: tests will probably not pass until #6118 is merged.

Like #6068, this is a new form intended for inclusion in DSL 1.0, but which was
overlooked during planning in #4688.

As with #6068, implementing `container :nested` now means we would not have the
benefit of the forward-compatibility transition code.  However, `nested_container` is
only used in 38 Casks in the main repo, so that is unlikely to cause a problem in practice.

The gain is consolidating two stanzas: `container_type` and `nested_container` are both
handled by `container`.  For a first-time Cask author, that seems more logical and regular.

We do lose the extensibility of `nested_container`, since `:nested` can only take a single
value.  However, it is unlikely that extensibility is needed, and there are workarounds.