Support C++ header file extensions (.hpp, .hh, .hxx, .h++) in LSP language detection

[Huijiro]

Problem

C++ projects commonly use header file extensions like .hpp, .hh, .hxx, and .h++ in addition to the standard .h extension. These are already supported by the clangd LSP server and clang-format formatter, but they're not currently mapped in the LSP language detection system.

Solution

Map these additional C++ header file extensions to the cpp language type in LANGUAGE_EXTENSIONS so they receive proper syntax highlighting and LSP support.

[github-actions]

This issue is related to several existing issues around C++ and LSP support:

• [FEATURE]: LSP support for choosing between multiple build directories (c++ / clangd) #7780: LSP support for choosing between multiple build directories (c++ / clangd)
• clangd LSP always fails #9521: clangd LSP always fails
• Clangd LSP set incorrect root and missing arguments #11617: Clangd LSP set incorrect root and missing arguments
• [FEATURE]: Formatter enhanced file extension matching #3431: Formatter enhanced file extension matching (demonstrates file extension pattern matching needs)

While these aren't exact duplicates, they represent related challenges in improving C++ language support and file extension detection in OpenCode. You may find context useful when implementing this feature.

[github-actions]

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