Adding customization types

[Etienne Prud’homme]

I was wondering if we could add several other customization types in
`wid-edit.el`.

One of the widget (or type), I would like to get is major-mode and
probably minor-mode too.  Is there any reason why it was never included?

Given we could know if a symbol is a major-mode, would it be okay if I
made a patch for that (to be included in `wid-edit.el`)?

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Etienne