Isolate `names(...)` in `event_data()` to prevent spurious updates

[dvg-p4]

Closes #2337 by wrapping names(session$userData$plotlyInputStore) with shiny::isolate().

I don't think there's any good way to write an automated test for this, but you can verify that the reprex I gave in the issue no longer produces the spurious updates, while the click data continues to be correctly displayed.

[dvg-p4]

@cpsievert is there anything else you'd like to see in the PR before approving the workflow / reviewing it?

[cpsievert]

Thanks! Will you also add a NEWS.md item?

[dvg-p4]

@cpsievert done!