(C, C++) Implement C2y and C++26 changes

[Eisenwave]

Describe the issue
The grammars for C and C++ do not yet include the latest C2y and C++26 features.

Additional context
This list is quite likely incomplete, but here are some of the changes:

• C++26 contracts (P2900) have been accepted (contextual keywords pre, post, and true keyword contract_assert)
• #embed is in C++, and on track to be added to C2y (add keyword in preprocessor mode)
• C++26 delete("reason") is possible, but delete is currently highlighted as a function name instead of a keyword in that situation
• C2y is on track to add defer as a feature (add keyword)
• C2y already supports break label and there are ongoing efforts to add this to C++; do we need dedicated label highlighting?

Additionally, the C23 type_BitInt is also provided as a C++ compiler extension by clang, and it might even become a proper keyword in C++ (though that remains to be seen). Anyhow, it should be a keyword even if it's non-standard.

See also https://en.cppreference.com/w/cpp/26