[EuiCodeBlock] Check and improve accessibility of expanding code blocks

[mgadewoll]

🔗 Caused by: elastic/kibana#196339

Description

When expanding an EuiCodeBlack it opens a fullscreen overlay with focus trap. This does not announce any change for screen readers currently.
We should ensure that the change is announced to users and DOM elements are marked semantically.

Additionally the EuiCodeBlock is focusable to provide means to scroll overflown content by keyboard when a height is set (related listbox pattern) but the element does not have any semantic role announced to users.

• <pre> elements don't have their own semantic roles
• <code> elements don't allow aria label-* attributes

Acceptance criteria

• (focusable) code blocks provide semantic information about what they are (code block)
• expanded code block is perceivable as such and the movement from the expand button to the expanded code block is clear