Add :others() pseudo-class

While AG supports most of uBO syntax, one operator not yet supported is [:others()](https://github.com/gorhill/uBlock/wiki/Procedural-cosmetic-filters#subjectothers). Even for uBO it was not meant to be used in official filters at first, but it turned out this is useful in some cases. Notablly I had a sucees in addressing persistent couner by (nsfw) `pus*yspace.c*m/p*ssyspace.n*t` with the operator in uBO and lack of this in AG made me to struggle more for AG filters. It can't be used if site owner doesn't mind breaking his own site, but as in the case mentioned, there are owners not willing to do so.