[water] even more persistent water emitters

Following up on #973, we may consider launching water/fx emitters somewhere in `__init__.py` of `wave_lang` and making them globally available. All further water dialect / fx emissions requests will then take an optional emitter field for overrides and default to using the global one. Similar to how MLIR context objects are exposed. May need mutual exclusion for thread-safety or a pool of emitters.

An alternative is to have a global placeholder and lazily initialize the emitter on the first request so we don't eat time loading things we don't need. Perhaps with an env variable forcing eager loads.