[gemini-kit] Clean up and fully convert to build API

[aomarks]

This is DRAFT because it's not fully building; need to do a couple things first:

• The build package's kit function isn't yet compatible with being imported into boards made with the old API (it is only compatible in terms of being loadable into a graph runner). Need to add that compatibility.

• vision is still used by a couple of example boards. Need to decide if we delete those too, or else convert them over to the other Gemini component.

Fixes https://github.com/breadboard-ai/breadboard/issues/2570