classHighlighter: map function(variableName) to tok-function

All cm6 themes define .tok-function but the classHighlighter had no entry
for tags.function(tags.variableName), so function-name tokens fell back to
tok-variableName (which themes leave unstyled).  This affected Typst function
calls (#func(...)) and would affect any future language that tags function
names with t.function(t.variableName).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

services/web/frontend/js/features/source-editor/extensions/class-highlighter.ts

@@ -23,6 +23,7 @@ export const classHighlighter = tagHighlighter([
     tag: [tags.regexp, tags.escape, tags.special(tags.string)],
     class: 'tok-string2',
   },
+  { tag: tags.function(tags.variableName), class: 'tok-function' },
   { tag: tags.variableName, class: 'tok-variableName' },
   { tag: tags.local(tags.variableName), class: 'tok-variableName tok-local' },
   {