Problem/Motivation
Node type config entities are mostly validatable using the typed config system, but not quite. There is at least one attribute (preview_mode) that needs a constraint or two on it. Once that's done, node types can be validated at the API level!
Proposed resolution
Add constraints to the preview_mode element of node.type.* in config schema.
User interface changes
None.
API changes
None.
Release notes snippet
TBD