Add "restart R?" yesno option when running `install_greta_deps()`

[njtierney]

• only shows up when interactive() returns TRUE (to avoid issues with scripting and restarting R without asking)
• or potentially add a restart = FALSE option in install_greta_deps() instead/in addition?

[njtierney]

Something like:

Would you like to restart R now: (yesno options)

with the restart R option, it could even have something that restarts R and runs library(greta)

.rs.restartR(afterRestartCommand = "library(greta)")

[njtierney]

Closed in #672