Resolve "Add global var to config, and use it in DiscloreDropdown and Listbox"

What does this MR do?

This issue closes #2798 (closed), which is Step 1 of the Epic &14444.

The task is to add the feature flag handling in gitlab-ui. This means, adding a setting to the Config, storing it in a global variable, and use the global variable in the components to opt-in to the improved hide heuristics feature of the Dropdown and Outside directive.

There are multiple advantages of using a feature flag described in this discussion in the epic. Most importantly for this MR, is that we prevented having to make code changes in ~160 .vue filed in the Gitlab repo to set the property improvedHideHeuristics to true. Instead, we will need one feature flag in the Gitlab repo, which sets the global variable to true in gitlab-ui.

Screenshots or screen recordings

Screen_Recording_2024-07-29_at_17.15.00

Integration merge requests

No integration required yet. I will add the link to the new setting in the GitLab Repository once it exists.

• GitLab: mr_url

Does this MR meet the acceptance criteria?

This checklist encourages the authors, reviewers, and maintainers of merge requests (MRs) to confirm changes were analyzed for conformity with the project's guidelines, security and accessibility.

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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.