feat(GlBroadcastMessage): Add `type` and `dismissible` props

What does this MR do?

Related issue: gitlab#368847 (closed)

Adds a couple of new props to GlBroadcastMessage to give us feature parity with the existing new/edit form preview.

Dismissible

A boolean which toggles the Dismiss button:

HAML generated	GitLab UI

Type

Adds the ability to use Notification type broadcast messages. Notifications have a different style applied and are not compatible with the theme or dismissible props. I copied the width restriction of 300px, but I did not include the style which positions the notification at the bottom-right of the viewport. I imitated the existing style, while keeping the existing GlBroadcastMessage padding:

HAML generated	GitLab UI

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines.
• GitLab UI's contributing guidlines.
• If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• If the MR changes a component's API, integration MR(s) have been opened in the following projects to ensure that the @gitlab/ui package can be upgraded quickly after the changes are released:
  • GitLab: mr_url
  • CustomersDot: mr_url
  • Status Page: mr_url
• Added the ~"component:*" label(s) if applicable.

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• Security reports checked/validated by a reviewer from the AppSec team

Accessibility

If this MR adds or modifies a component, take a few moments to review the following:

• All actions and functionality can be done with a keyboard.
• Links, buttons, and controls have a visible focus state.
• All content is presented in text or with a text equivalent. For example, alt text for SVG, or aria-label for icons that have meaning or perform actions.
• Changes in a component’s state are announced by a screen reader. For example, changing aria-expanded="false" to aria-expanded="true" when an accordion is expanded.
• Color combinations have sufficient contrast.