Initial release of the standalone Treescope pretty-printer.

Treescope was originally designed as the pretty-printer for the Penzai neural network library, but has been moved to its own repository so that it can be installed and used independently. See the commit history for Penzai for earlier versions of Treescope.

Changes relative to the penzai.treescope submodule of Penzai release 0.1.5:

• Moved all functions and methods out of the penzai.treescope namespace and into treescope.
• Renamed extension protocol method from __penzai_repr__ to __treescope_repr__.
• Added a global handler registry, to enable registering handlers for existing types.
• Redesigned the array visualization system in terms of an NDArrayAdapter protocol, enabling it to support non-JAX arrays.
• Added handlers for PyTorch tensors and PyTorch neural network modules.
• Moved implementation details of the internal intermediate representation into an _internal subpackage, and added new public construction methods for various rendering components.
• Removed some complex features that were rarely used (copy-paste fallbacks and between-node hyperlinks).