DSL: add dsl stanza, defining DSL specification version for a Cask

[rolandwalker]

References: #4688

Renamed dsl from earlier proposed stanza spec.

I remain conflicted about adding boilerplate to the Cask language. Simplicity is
what makes Casks amenable to first-time authors, and first-time authors are the
principal drivers of this project.

However, the technical advantage of specifying the DSL version is pretty hard to deny
if we merge #3066 — which is also seeking simplicity, in the form of very straightforward
bookkeeping.

The alternatives to #3066 are: binary serialization (which will break), or creating
a new log format which largely duplicates the info in the Cask DSL — which requires
much more new code. Also, both alternatives would reduce transparency to the
user.

[rolandwalker]

Because this information has been moved to the header in #5365, a dsl stanza is no longer needed.