[tsp-client] Investigate support for 2 emitters in emitter-package.json

If we specify 2 emitters in emitter-package.json we would need a way to signal which emitter is always the default emitter. Specs that require the other emitter should specify it through their tsp-location.yaml file. Special case for init, where we'd need a special flag to signal what emitter to use if it's not the default emitter.