Add default value for regexFindMode setting.
Add default value for regexFindMode setting.

Because we haven't had a default value for this setting, we never sync
it, which means -- when its not at its default value -- it isn't picked
up in content scripts by the new settings system.

Fixes #1731.

douglas-larocca/vimium,hhsue/vimium,willsALMANJ/vimium,justalittlenoob/vimium,philc/vimium,bkudria/vimium,Lw-Cui/vimium,poacher2k/vimium,gdh1995/vimium,cyrixhero/vimium,PickRelated/vimium,zhangmin510/vimium,tuchangwei/vimium,justalittlenoob/vimium,justalittlenoob/vimium,smblott-github/vimium,borgified/vimium,cyrixhero/vimium,douglas-larocca/vimium,smblott-github/vimium,elegantwww/vimium,Robinson10240/vimium,hhsue/vimium,Robinson10240/vimium,PickRelated/vimium,dimatter/vimium,tuchangwei/vimium,zhangmin510/vimium,PickRelated/vimium,borgified/vimium,elegantwww/vimium,VPashkov/vimium,zhangmin510/vimium,csmalin/vimium,VPashkov/vimium,gdh1995/vimium,bkudria/vimium,mrmr1993/vimium,philc/vimium,Aleroniponi/vimium,mrmr1993/vimium,poacher2k/vimium,douglas-larocca/vimium,elderbas/vimium,PrestanceDesign/vimium,Lw-Cui/vimium,philc/vimium,hhsue/vimium,mrmr1993/vimium,bkudria/vimium,dimatter/vimium,csmalin/vimium,decaffeinate-examples/vimium,Aleroniponi/vimium,decaffeinate-examples/vimium,tuchangwei/vimium,willsALMANJ/vimium,csmalin/vimium,gdh1995/vimium,elderbas/vimium,smblott-github/vimium,cyrixhero/vimium,decaffeinate-examples/vimium,willsALMANJ/vimium,Robinson10240/vimium,elderbas/vimium,PrestanceDesign/vimium,elegantwww/vimium,borgified/vimium,PrestanceDesign/vimium,VPashkov/vimium,Lw-Cui/vimium,dimatter/vimium,poacher2k/vimium,Aleroniponi/vimium