[Component amendment]: Button - processing

## Component/pattern to amend

`is-processing`

## Visual

https://vanillaframework.io/docs/patterns/buttons#processing

## Context

`is-processing` is used to alter a disabled button, so it doesn't make sense for it to be applied to an enabled button. For this reason, I think it's worth considering that `is-processing` should also extend `is-disabled`.

Start by searching the framework for the `is-processing` and `is-disabled` button classes, then compare their behavior with the processing button documentation at the linked URL. Done means processing buttons consistently inherit the disabled behavior without affecting enabled buttons, with the documentation reflecting the result.

Written by the indexing model from the issue text.