dtlib: type annotate DT fields and public methods

Now that all the other code it depends on is annotated, we can finish
up the type annotation of this module in the main DT class.

It's not worth it to try to annotate the private methods (the ones
that begin with '_'). Most of these are low level lexing helpers that
aren't particularly amenable to static type checking, because the type
of a token's value is often dependent on the token ID in ways that
static type annotations are not well equipped to capture.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>