Ignore disabled checkboxes

[guillaumebriday]

fix https://github.com/stimulus-components/stimulus-checkbox-select-all/issues/9

[adrienpoly]

This prevents checking / unchecking disabled fields

Now it raises one more question for the intermediate state.

With this implementation checking all and having one disabled (unchecked) input will keep the intermediate state.

Example [-] check all / uncheck all [x] option 1 [ ] option 2 Disabled [x] option 3

Not sure if there is an official way to handle that

My personal feeling is that if I click check all it should remove the intermediate state even if some disabled inputs are not checked