isocpp.xml: Highlight raw string delimeters (+corresponding tweaks to default themes)
Highlighting delim in R"delim( content )delim"
helps in differentiating the raw string delimiter
from the actual content and also makes it easier
to spot mismatching delimiters.
With my unscientific testing among the color themes shipped with kate by default, dsOthers has good enough balance between subtlety and contrast (with normal strings and the content of the raw string) compared to other candidate styles (dsSpecialString, dsRegionMarker, dsConstant, dsNormal, dsComment, dsAnnotation)
Before and After screenshots for Breeze:
Edited by Jan Paul Batrina