Inform that project `.Renviron` may take precedens when using `edit_r_environ("user")`

[netique]

This occurred to me as I was trying to edit .Renviron in an RStudio project that was equipped with project-specific .Renviron. Naturally, I was editing user .Renviron as this is the default for edit_r_environ() only to see that it had no effect. You could detect if there is project-specific .Renviron and issue a bullet point in edit_r_environ() message to remind the user that different .Renviron may be taken into account.

[jennybc]

Notes for anyone who tackles this, e.g. at tidy dev day:

There are several existing functions that start with challenge_ that would be good inspiration for how to alert the user to this situation.