Source/Header switching using shell expansion

[rcorre]

Using the new shell expansions, I hacked up source/header switching for .cpp files. It's not quite as accurate as using an LSP (it assumes the file stems are the same), but given that's not practical right now in helix, this works pretty well for me:

[keys.normal.g]
H = """
:o %sh{
case '%{buffer_name}' in
  *.h)
    echo '%{buffer_name}' | sed 's/.h$/.cpp/'
    ;;
  *.cpp)
    echo '%{buffer_name}' | sed 's/.cpp$/.h/'
    ;;
esac
}"""