feat(GlDrawer): Emit "opened" event in transition after-enter hook

What does this MR do?

Closes Add @opened event to drawer for when it complet... (#2093 - closed)

Why is this tested with Cypress?

In Jest it is tricky to test a Vue event like after-enter that is fired after a CSS transition finished. Even mocking the <transition> component didn't work, because this approach failed with the Vue 3 test suite.

Cypress tests run in a real browser where we can observe an effect of the event listener. This needed a small tweak to the storybook used by Cypress.

Screenshots or screen recordings

No visual change.

Integration merge requests

Only adds a new $emit. No integration branches required.

Does this MR meet the acceptance criteria?

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Accessibility

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