Implement named backreference support

[hamishknight]

Named backreferences e.g (?<x>)\k<x> are not currently supported, but it seems like we ought to be able to lookup the capture index from the capture list, and emit it the same as a numbered backreference (i.e \1).

[hamishknight]

@milseman Does it seem reasonable to support this for 5.7? It would also be nice to support relative numbered captures (i.e \k<-1>), but they could probably be future work if needed (I believe we could back deploy as a regular numbered capture).

[milseman]

Relative is less important than named. Let's try to get named in