feat: Add segmented control

Closes #241 (closed)

This MR adds segmented control to gitlab-ui This component is essentially a wrapper around b-form-radio

Note for reviewer:

• It was agreed not to make "multiple choices control" at gitlab-org/gitlab-services/design.gitlab.com#269 (comment 197752002)

• design.gitlab.com states

  There must always be one option active

  This is a difference in behavior from b-form-radio and is not strictly followed in current gitlab-ce codebase. Proper warnings were added (along with tests) to improve developer experience and help us in ensuring consistency

Also since we're just going with "all default" styling, I believe it's also safe to close #242 (closed) as redunant when this one is merged

Related issues:

• gitlab-org/gitlab-services/design.gitlab.com#269 (closed)