Button component - group-button - create Vue component
### Problem
Create Vue component for `group-button`
### Proposal
- Details can be derived from specs.
- Individual buttons can be different types (primary, secondary, tertiary)
#### Attributes
Types:
- Primary
- Style: info
- Style: success
- Style: warning
- Style: danger
- Secondary
- Style: info
- Style: success
- Style: warning
- Style: danger
- Tertiary (default)
- Style: default
States:
- Default
- Hover
- Active
- Focus
- Disabled
Sizes:
- Small
- Medium (default)
- Full width
### Links
* [Documentation](https://design.gitlab.com/components/buttons/#tertiary)
* [Specs](https://gitlab-org.gitlab.io/gitlab-design/hosted/design-gitlab-specs/buttons-spec-previews/)
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