Externalize constraints

[edgarmueller]

When using the library in order to build up a JSON schema AST using constraints (e.g. to be emitted as JSON), one needs access to internal packages.

[edgarmueller]

Also, before externalizing, one should replace schemaTypeAsString with an appropriate boolean for each schema type. Currently, one can provide invalid values for the type keyword since it expects a string.

[edgarmueller]

Another related issue: booleanWriter does delegate directly to anyConstraintWriter as it should be and hence always includes the type in the serialization.