Allows the split dropdown prop to work

I need split button support for one of my deliverables but because of the way we add the caret, it's proving difficult.

1. Adding the split prop hides the caret in the regular part of the dropdown
2. We pass the split prop directly through to the bootstrap dropdown, allowing it to be split.
3. Show the original bootstrap-vue caret till we can find a way of passing the usual caret.

Because the caret is displayed using CSS on bootstrap Vue, and is an injected <i> component on our dropdowns, I've had to use the CSS caret on the split part of the dropdown. I've looked at the render method in the Bootstrap Vue source but it looks like it's a closed off component.

Ideally, we'd have the same caret for split dropdowns (and no CSS in this component), but this is a boring solution fix that I need for https://gitlab.com/gitlab-org/gitlab-ee/issues/9224 until we can do this properly.

~feature