Extend `concat-examples` support

[sellout]

concat-examples contains a number of useful categories, but they’re not quite usable by Categorifier in general.

This sets up a pattern for supporting them and implements it for the ConCat.Syntactic category. It defines the plugin-specific instances for them (and moves existing ones out of test-specific code) and also defines concat-extensions and unconcat instances.

Syntactic is a trivial category, but the pattern should work across others, which can be integrated over time.