Relax the prohibition on setting LC_ALL

[jennybc]

Motivated by some work I'm doing in reprex.

It would be nice to relax this prohibition in [with|local]_locale():

https://github.com/r-lib/withr/blob/master/R/locale.R#L6-L8

Have discussed in Slack with @jimhester and we think it's not that difficult and is worth doing. Capturing that convo here.

From the docs:

Note that setting category "LC_ALL" sets only categories "LC_COLLATE", "LC_CTYPE", "LC_MONETARY" and "LC_TIME".

You can't just take the output of Sys.getlocale() and roundtrip it straight into Sys.setlocale(). Instead, it will look more like:

... detect LC_ALL and convert it to stats::setNames(rep(my_new_locale, 4), c("LC_COLLATE", "LC_CTYPE", "LC_MONETARY", "LC_TIME"))