Formatting: Prevent wp_slash from returning non-strings as strings.
If a bool/float/int is passed into wp_slash it will be coerced into a string.
This changes the behavior to only slash strings. At the same time, handles recursion a little nicer by calling array_map for arrays.
Fixes #42195, #24106.
Props johnbillion, andizer, jrf, ryotasakamoto, SergeyBiryukov, donmhico, TobiasBg, markoheijnen, ryan, nacin, devesine, whyisjake.