Expose what *Help > Check for Updates* does

[jennybc]

devtools::dev_sitrep() gives a "situation report" on the user's dev tooling stack.

If we detect we're in RStudio, we try to detect if someone is running an outdated version of RStudio and suggest how to update. I think we're just emulating Help > Check for Updates.

Our method recently broke due to the change in RStudio version numbering (r-lib/devtools#2397).

It would be great if this operation (get user's current version, hit the appropriate API or URL, parse and interpret those results) could be exposed in rstudioapi.