feat(BaseDropdown): Adds "right-start" placement

What does this MR do?

This change allows a new placement option for dropdowns so they can support "dropright" placements.

This option allows building UIs such as the pipeline job groups.

Why?

We are migrating our pipelines to use gl-disclosure-dropdown and they have the following placement position:

Usage

<gl-base-dropdown :placement="right-start" ...>
  ...
</gl-base-dropdown>

<gl-disclosure-dropdown :placement="right-start" ...>
  ...
</gl-disclosure-dropdown>

Screenshots or screen recordings

In situations where space is restricted, the dropdown allows for other right/left positions:

Integration merge requests

• GitLab: mr_url
• CustomersDot: mr_url
• Status Page: mr_url

Does this MR meet the acceptance criteria?

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Conformity

• Code review guidelines.
• GitLab UI's contributing guidelines.
• 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 (see integration merge requests above).
• 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.