Fix #770: clerk/row metadata can't be added to a def

[borkdude]

Fixes:

• docstring should say: :var-from-def? instead of :nextjournal.clerk/var-from-def
• unwrapping happens on return value (after applying function viewer)

Added tests for:

• A function viewer that gets applied to a var-from-def value
• A function that opts out of unwrapping

It's this part in particular that crashed before:

 (testing "function viewer with var-from-def"
      (is (= [{:nextjournal.clerk/var-from-def #'nextjournal.clerk.viewer-test/my-test-var2, :nextjournal.clerk/var-snapshot 1}]
             (apply+get-value {:nextjournal/value {:nextjournal.clerk/var-from-def #'my-test-var2
                                                   :nextjournal.clerk/var-snapshot 1}
                               :nextjournal/viewer v/row}))))