New KDE syntax-highlighting feature: multi-stack context

[jonathanpoelen]

There will be a new syntax for contexts, which consists of specifying multiple contexts in the context attribute, separating them with !.

<RegExpr String="(.)" attribute="Symbol" context="A!B"/>

A and B are stacked, with the last one on the list being the active one.
You can also specify a syntax at the same time as the context: Ctx1##Syntax1!Ctx2!Ctx3##Syntax1

Captures with RegExpr are only effective on the last item in the list (B here).

https://invent.kde.org/frameworks/syntax-highlighting/-/merge_requests/747

[jgm]

Once it is added and syntax definitions start to use it, let us know and we can update skylighting accordingly.