Enable auto-completion

Hi @timtreis,

I never had access to code auto-completion using `spatialdata-plot`. I guess it's not specific to me, but likely related to the way `_register_accessor` is used, correct?

I guess you already investigated why the auto-completion doesn't work, so you may already have insights regarding this. Do you think it's something that could be potentially fixed, or is it not planned at all?