Add a .h header file icon instead of using the C icon

[sean-mcmanus]

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Actual: For .h files, the seti-ui theme is using the C icons. This is incorrect/misleading when .h files actually represent C++ files, which is commonly the case.

Expected: For .h files, a "H" header file icon should be used that is valid for C or C++. This would match the behavior of Visual Studio.

An alternative would be to have a C++ specific header for .hpp files, which is what the vscode-icons extension does, but I don't think that is necessary.

[sean-mcmanus]

The Studio Icons extension also handles this well (the icon looks like VS).

[orta]

Great idea, PRs welcome for this!