[Bug] `'dirvish-open-with-programs` has an incorrect `'custom-type` property

Thank you for the bug report

[X] I am using the latest version of dirvish related packages.
[X] I checked CUSTOMIZING|EXTENSIONS.
[X] You may also try reproduce the issue using clean environment and minimal configurations with the command emacs -Q.

Bug description

'dirvish-open-with-programs has an incorrect 'custom-type property. This prevents it from being customized via the customize interface, and fails programmatic type-checking (see the Anything else section below).

Steps to reproduce

(customize-variable 'dirvish-open-with-programs)

and see that no customization buttons specific to 'dirvish-open-with-programs show up in the customize buffer and the following message appears in the message buffer:

mapcar: Wrong type argument: symbolp, (repeat string)

(widget-convert (get 'dirvish-open-with-programs 'custom-type))

and see that an error is signaled (my erro callstack is provided below).

Expected behavior

Customize buffer displays properly;

widget-convert works.

OS

Linux

Emacs Version

Emacs Configurations

Vanilla

Error callstack

Debugger entered--Lisp error: (wrong-type-argument symbolp (repeat string))
  widget-convert(((repeat string) :tag "File extensions"))
  mapcar(widget-convert (((repeat string) :tag "File extensions") ((repeat string) :tag "External command and args")))
  widget-types-convert-widget((cons :format "%v" :args (((repeat string) :tag "File extensions") ((repeat string) :tag "External command and args"))))
  widget-convert((cons :format "%v" ((repeat string) :tag "File extensions") ((repeat string) :tag "External command and args")))
  mapcar(widget-convert ((cons :format "%v" ((repeat string) :tag "File extensions") ((repeat string) :tag "External command and args"))))
  widget-types-convert-widget((editable-list :inline t :args ((cons :format "%v" ((repeat string) :tag "File extensions") ((repeat string) :tag "External command and args")))))
  widget-convert((editable-list :inline t (cons :format "%v" ((repeat string) :tag "File extensions") ((repeat string) :tag "External command and args"))))
  mapcar(widget-convert ((editable-list :inline t (cons :format "%v" ((repeat string) :tag "File extensions") ((repeat string) :tag "External command and args")))))
  widget-types-convert-widget((alist :key-type ((repeat string) :tag "File extensions") :value-type ((repeat string) :tag "External command and args") :args ((editable-list :inline t (cons :format "%v" ((repeat string) :tag "File extensions") ((repeat string) :tag "External command and args"))))))
  widget-convert((alist :key-type ((repeat string) :tag "File extensions") :value-type ((repeat string) :tag "External command and args")))
  eval-expression((widget-convert (custom-type 'dirvish-open-with-programs)) nil nil 127)

Anything else

I use the #'custom-type and #'custom-value-satisfies-type-p functions of cus-edit-plus (https://github.com/emacsmirror/cus-edit-plus/blob/99b16cb95e1b892e15e3091f124e0be6dd914e16/cus-edit%2B.el#L1073) to type-check any variables I set (if they have a 'custom-type property). This allows me to catch many type errors like this one. Dirvish may want to do something similar.

alexluigit / dirvish