Registry for PrepareError

[divergentdave]

draft-priebe-ppm-dap-reportauth (no I-D submitted yet) wants to define a new PrepareError type, but we do not have a registry for this, only the enum definition. In conjunction with setting up the necessary policy for PrepareError extensibility, we may want to expand the encoded size from one byte to two bytes. We are already using 4% of codepoints for DAP itself, and additional headroom would make managing this namespace easier.