A block of authored prose.
text is a template in the subset Riddler.Template admits, so a block of
copy can read back what the visitor just told us. Nothing else about it is
this type's to check: the template is compiled by the document, for every
template field of every type, by the same code an editor calls.
iex> Riddler.Screens.Type.Text.validate(%{type: "text", key: "plan_intro", text: "Nothing is charged today."})
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