core: expose default and desktop configs on index.js

[adamraine]

I think it would be useful to have these two at the top level api.

Ref #14048

[brendankenny]

This seems fine but can you explain the use case? This also came up when we were discussing lightrider-entry configs: extending the default configs is the express purpose of flags and the config extends field.

[adamraine]

Having the desktop config in an accessible place is better than telling people to pull it in from lighthouse/core/config/desktop-config.js.

I can't think of a specific use case for having the default config exposed here, but there isn't a reason to not include it either.

[brendankenny]

Makes sense. We should finally make extends work with the other presets (desktop/perf/experimental, etc) and maybe even arbitrary extends config chains, but sounds good.