Several libcpp headers lack file extensions. Added a first line pattern
for recognition.


https://github.com/llvm/llvm-project/blob/main/libcxx/include/random)](https://github.com/llvm/llvm-project/blob/main/libcxx/include/random

Before:
<img width="1229" height="512" alt="before"
src="https://github.com/user-attachments/assets/fe62f904-73c9-494e-adf5-e5f259fab611"
/>

After:
<img width="1229" height="512" alt="after"
src="https://github.com/user-attachments/assets/a68b4242-61b7-4dba-a12e-9424be4b06dc"
/>

Release Notes:

- Added first-line C++ language detection for extensionless libcpp
headers.