Add default settings for provider

[brichet]

Also since we ship a couple of providers by default, maybe it could make sense to also ship a set of "good defaults", to have the best experience of @jupyterlite/ai out of the box. For example that could mean choosing some models by default for each of the providers that we know work well. Of course they would still be customizable via the settings.

Originally posted by @jtpio in #85 (comment)

[jtpio]

Of note, some providers have two entries for setting the API keys and the model, which may be confusing to users: