Support enums

[silasdavis]

This seems like the most obvious/useful place to start with types that need to be pulled in from Postgres. Enum types and their values need to be read from the Postgres catalog and passed along as additional types from getTypes in the query package. These types can be merged in with the type mapping in #51 which can generate a corresponding typescript enum declaration as a TypeAlias and output that in the header of the generated file.

A similar approach can probably be used for composite types.