Make DEFAULT_TRAVERSE_LIMIT larger or configurable

[sangotaro]

Describe the feature

Hello, thanks for this great library!

I'm dealing with messages larger than the current default traversal limit of 64 MB (64 << 20), and it's causing errors.
In a CLI environment, it's not feasible to modify message._capnp.traversalLimit directly.

It would be very helpful if you could:

1. Increase the default DEFAULT_TRAVERSE_LIMIT (for example, to 128MB), or
2. Provide a way to configure the traversal limit (e.g., via a constructor option).

If this is something you'd consider, I'd be happy to help with a PR.
Thank you for your time and consideration!

Additional information

• Would you be willing to help implement this feature?