This moves the protobuf module from experimental to deprecated
Rationale:
* This is very seldom used, if used at all
* Not fully fleshed out (this really should be paired with a protoc-gen-pkl, which doesn't exist)
* There is a ProtoJSON wire format that we can already render to
This enables defining declarative key and/or value transformations in
cases where neither `Class`- nor path-based converters can be applied
gracefully. It is also the only way to express transforming the
resulting property names in `Typed` objects without applying a converter
to the entire containing type, which is cumbersome at best.
SPICE: https://github.com/apple/pkl-evolution/pull/26
Daniel Chao