4940: Add support for marking doctest items as distinct from normal code r=ltentrup a=Nashenas88 This adds `HighlightTag::Generic | HighlightModifier::Injected` as the default highlight for all elements within a doctest. Please feel free to suggest that a new tag be created or a different one used.  Fixes #4929 Fixes #4939 Co-authored-by: Paul Daniel Faria <Nashenas88@users.noreply.github.com> Co-authored-by: Paul Daniel Faria <nashenas88@users.noreply.github.com>