Fix incorrect first line detection for libstdc++ files (#48351)

feeiyu created 

https://github.com/zed-industries/zed/pull/48250#issuecomment-3846774856

To ensure the fix covers all affected files, I have checked all C++
files without extensions using the following command:
```bash
find /usr/include/c++ -type f | grep -v '.h' | grep -v '.cpp' | grep -v '.tcc' | xargs head -1 | zed -
```

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Release Notes:

- N/A

Change summary

crates/languages/src/cpp/config.toml | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

Detailed changes

crates/languages/src/cpp/config.toml 🔗 

@@ -2,7 +2,7 @@ name = "C++"
 grammar = "cpp"
 path_suffixes = ["cc", "hh", "cpp", "h", "hpp", "cxx", "hxx", "c++", "h++", "ipp", "inl", "ino", "ixx", "cu", "cuh", "C", "H"]
 line_comments = ["// ", "/// ", "//! "]
-first_line_pattern = '\/\/\s*(<.*>\s)?-\*-\s*C\+\+\s*-\*-'
+first_line_pattern = '^//.*-\*-\s*C\+\+\s*-\*-'
 decrease_indent_patterns = [
   { pattern = "^\\s*\\{.*\\}?\\s*$", valid_after = ["if", "for", "while", "do", "switch", "else"] },
   { pattern = "^\\s*else\\b", valid_after = ["if"] }