pvr: csbgen: Add typing information

pvr: csbgen: Add typing information

This is generally PEP-484 stuff, but there is one functional change.
The base class Node needed to have an add() method to allow typed
dynamic dispatch. This could have been decorated @abstractmethod, but
that would require an error-raising implementation on all leaf-type
nodes. Instead, I added a base implementation that just errors out with
information from the subclass instance.

As a simple optimization, subclass implementations of add() (instead of
raising the same (or similar) error) now call super().add() in the
case of invalid child nodes.

Signed-off-by: Matt Coster <matt.coster@imgtec.com>
Reviewed-by: Frank Binns <frank.binns@imgtec.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16884>