Allow `no-caret` option to actually hide the caret

Allows no-caret to be used in <gl-dropdown>. I added this to help avoid some weirdness with split dropdowns (coming in a future MR)

The no-caret param exists in bootstrap-vue but we override it to provide our own in GitLab UI. This Allows us to use it again.

• Conditionally renders the chevron depending on the param
• Creates and example and page