[stm] First step to move interpretation of Undo commands out of the classifier.

The vernacular classifier has a current special case for "Undo" like
commands, as it needs access to the document structure in order to
produce the proper "VtBack" classification, however the classifier is
defined before the document is.

We introduce a new delegation status `VtMeta` that allows us to
interpreted such commands outside the classifier itself.

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