cuda-mode should be derived from prog-mode

[eyal0]

This fixes #10

[eyal0]

To test this, I deleted the cached, compiled version of cuda-mode and patched the file. Here's what a cuda file with cuda-mode and spell-fu enabled used to look like:

Now I patch in the change into cuda-mode.el and delete cuda-mode.elc (not sure if that is needed)

emacs ~/.emacs.d/elpa/cuda-mode-20201013.2230/cuda-mode.el
# patch the file
rm ~/.emacs.d/elpa/cuda-mode-20201013.2230/cuda-mode.elc

Then I reopen my test file:

Notice that the word blarg is now a spelling error only in the comment. code is not spell checked by spell-fu, only comments are. This is correct behavior for spell-fu.

https://github.com/emacsmirror/spell-fu
@ideasman42

[eyal0]

This is what spell-fu is doing to ignore all but comments in prog-mode-derived modes:

https://github.com/emacsmirror/spell-fu/blob/master/spell-fu.el#L1062-L1068