Introduce the Node-level Datatypes API (#3462)

Introduce the Node-level Datatypes API (#3462)

This adds classes corresponding to the Node-level Datatype API "DType", which is a specification for a datatype type. It does not enable the use of this layer yet. A followup PR will update the Expr-level Datatype to use the Node-level code, which is currently verified to be functional on this branch: https://github.com/ajreynol/CVC4/tree/dtype-integrate. Futher PRs will make the internal (Node-level) code forgo the use of the Expr-layer datatype, which will then enable the Expr-layer to be replaced by the Term-layer datatype.

Most of the documentation for the methods in DType/DTypeConstructor/DTypeSelector was copied from Datatype/DatatypeConstructor/DatatypeConstructorArg.