vim-patch:77099ed: runtime(cpp): add C++26 lexical constructs to syntax highlighting

Add a guarded "C++ 26 extensions" block (cpp_no_cpp26) covering new
lexical surface introduced since C++23:

- [[ ... ]] attributes as a region, so P3394 annotations carrying a
  value expression (eg [[=foo{1}]]) no longer trip cErrInBracket on
  their braces/parens. A \w\@1<! look-behind keeps it from matching a
  subscripted immediately-invoked lambda (arr[[]{...}()]).
- ^^ reflection operator (P2996).
- [: :] splice brackets (P2996).
- contract_assert keyword (P2900).

Add input/cpp_cpp26.cpp exercising these constructs with screendumps,
and update dumps/cpp_noreturn_00.dump for the new [[ ]] attribute
delimiter highlighting.

closes: vim/vim#20577

https://github.com/vim/vim/commit/77099ed6b3b35b6340fd4945ba219020034a9182

Co-authored-by: Gareth Lloyd <gareth@ignition-web.co.uk>

runtime/doc/syntax.txt

@@ -777,6 +777,7 @@ cpp_no_cpp14		don't highlight C++14 standard items
 cpp_no_cpp17		don't highlight C++17 standard items
 cpp_no_cpp20		don't highlight C++20 standard items
 cpp_no_cpp23		don't highlight C++23 standard items
+cpp_no_cpp26		don't highlight C++26 standard items
 
 
 CSH							*ft-csh-syntax*

runtime/syntax/cpp.vim

@@ -110,6 +110,15 @@ if !exists("cpp_no_cpp23")
   syn keyword cppType		float16_t float32_t float64_t float128_t bfloat16_t
 endif
 
+" C++ 26 extensions
+if !exists("cpp_no_cpp26")
+  " attribute [[ ... ]] with optional value expr, eg [[=foo{1}]]
+  syn region cppAttribute	matchgroup=cppAttributeBracket start="\w\@1<!\[\[" end="\]\]" contains=TOP,@Spell
+  syn match cppReflect		"\^\^"
+  syn match cppSpliceBracket	"\[:\|:\]"
+  syn keyword cppStatement	contract_assert
+endif
+
 " The minimum and maximum operators in GNU C++
 syn match cppMinMax "[<>]?"
 
@@ -134,6 +143,9 @@ hi def link cppString		String
 hi def link cppNumber		Number
 hi def link cppFloat		Number
 hi def link cppModule		Include
+hi def link cppAttributeBracket	Special
+hi def link cppReflect		Operator
+hi def link cppSpliceBracket	Special
 
 let b:current_syntax = "cpp"