Delegate to the maktaba#selector API

[malcolmr]

My understanding of google/vim-maktaba#208 is that we forked most of the guts of this project into Maktaba, as maktaba#selector#Create.

David's comment in google/vim-maktaba#209 finished:

Next step will be to gut the vim-selector implementation once users have had some time to update maktaba and make it delegate to maktaba for all the non-legacy implementation, and then to mark the whole vim-selector plugin deprecated.

I think we still have that to do, here.

[dbarnett]

Actually, the backwards compatibility precautions are probably more trouble than they're worth. We could just add a deprecation notice, make sure our own usages are updated to the maktaba#selector# version, and archive the repo.

[dbarnett]

Let's also leave a note in a stub README here in the archived repo if there are any compatibility tricks like including an explicit <CR> action, or checking out a particular maktaba version temporarily to avoid newer API changes.