fix(GlDropdown): Make dropdown more accessible

What does this MR do?

This MR makes GlDropdown more accessible. The impetus was the use case where we want to show a dropdown with a vertical ellipsis and no text—currently in this scenario there is no accessible name for the dropdown button.

This MR:

• Adds the ability to make the dropdown button text visually hidden
• Hides icons from screen readers as they are visual decoration
• Adds a story for an icon only dropdown
• Adds more Storybook knobs for the dropdown component

The naming of the text-sr-only prop was inspired by how the Bootstrap Vue modal component implements an sr-only title: https://bootstrap-vue.org/docs/components/modal#comp-ref-b-modal-props

Screenshots

before	after

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
  • Customers Portal: 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