This change fixes two things with the list edit widget, which is used for the diffviewer syntax highlighting file type override configuration. 1. In the case that the widget is used within a ConfigForms page, the 30em width for input fields was causing the widget to bust out of the 60em width of the config forms page. ConfigForms is desperately in need of a comprehensive redesign, but for now I've added some style overrides to remove the 30em width and instead apply `flex-grow: 1`. 2. This change also swaps out the `btn` class for `ink-c-button`, giving us a better display when running in dark mode. While doing this, I've removed `role="button"` from the element, since that's implicit for `<button>` tags. Testing Done: - Viewed the widget in the context of a ConfigForms page and saw that it was now fitting nicely into the width of its container, and the "Add item" button looked nice in both light and dark modes. - Ran js-tests.