EventBrite Block: Focus not returning to invoking element on modal close (A11Y)

In testing the accessibility of this block, I found this issue where - on modal close, the focus isn't returned to the invoking element. Thus keyboard-only users will have a hard time navigating this.

#### Steps to reproduce the issue

1. Add an Eventbrite Checkout block to a post or page and set the "Embed Type" to "Button & Modal".
2. View post/page.
3. Click on the "Register" button to open the modal.
3. Click the "close" button or the escape key.
4. See that the button is not focused.

#### What I expected

Modal-invoking element has `:focus`.

#### What happened instead
Modal-invoking element does not have `:focus`.

Start by locating the Eventbrite Checkout block's Button & Modal implementation and reproduce the focus behavior described in the issue. Trace the close-button and Escape-key paths, then verify that closing the modal returns focus to the invoking Register button for keyboard users.

Written by the indexing model from the issue text.