Support "close" requests to data explorer R backends

[wesm]

R side of #2304

[jmcphers]

The R backend already supports close(), but it does not self-close when the data object is overwritten.

[jmcphers]

This is ready to verify, though it will be hard to do since the UI doesn't currently tell you when you're disconnected. #2516

[juliasilge]

Now done in Positron 2024.04.0 (Universal) build 103

[wesm]

We'll have to circle back once the UI has a "disconnected" state to make sure this works as expected.