bug#52523: 29.0.50; defcustom :type 'key-sequence not following key-valid-p format

[Juri Linkov]

> Now that we prefer the key-valid-p format, we should think about what to
> do about defcustom as well.  We currently have that:
>
>     (defcustom foo "C-a" "" :type 'key-sequence)
>
> Followed by `M-x customize-option RET foo RET' leads to a customize
> buffer where this is displayed as "C - a".  IOW, there is only support
> for the legacy format "\C-a".
>
> We can't just change this in a backwards incompatible way, but perhaps
> we should document it as deprecated and introduce a new format for the
> new type?

Indeed, a new type would be preferable.  How about

  :type 'kbd