Add support for WriteRegMultiStr

[idleberg]

This is a weird design choice on the NSIS side of things, since this keyword always requires the /REGEDIT5 switch. I dug a bit deeper and stumbled across (this comment), indicating that this might change at some point. However, nothing has changed in that regard in the past 9 years.

I'm not sure what the desired way is to deal with this. NSIS development is slow at best. Should we go for a simpler API and consider a breaking change in the future, or anticipate a change right now?