Use UnderlyingType for *Type exports

[bengreenier]

There is still some inconsistency between what a *Def stores, and what a *Type represents. To avoid this, it's probably better to use UnderlyingType from ref-napi rather than generating it ourselves. E.g:

type MyStructType = UnderlyingType<typeof MyStructDef>;

instead of

type MyStructType = StructObject<MyStruct>;

since the TS typing and the coerced ref-napi NamedType may not align exactly.