Include AI impact analytics panels in the built-in visualization list

Problem

The panels used in the AI impact dashboard are not shown when creating a custom dashboard, which makes it hard for users to add them to their custom dashboards.

Solution

Add AI_IMPACT_DASHBOARD_VISUALIZATIONS to the list of built-in visualizations.

We will need conditionally show these visualizations when the AI impact dashboard is available: https://gitlab.com/gitlab-org/gitlab/-/blob/45562b25c5dc445f90beef3366d6c95b82372ac3/ee/app/models/product_analytics/dashboard.rb#L129