kak-lsp uses these faces to mark errors inside the buffer, instead of the Error
face which is much more jarring, and which does not have an associated warning
face. Since the :spell command marks errors inside the buffer, it's also updated
to use this new face.
Adding these faces to Kakoune makes it more likely that colorschemes will
automatically do the right thing when used with kak-lsp, and makes it possible
to use a subtle appearance (like curly underlines) for in-buffer errors while
keeping Kakoune errors bold and jarring as they should be.
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