feat(core/menu): implement a11y features for menu and burger-menu

[jg0815]

Pull request checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)
• Build (yarn build) was run locally and any changes were pushed
• Unit tests (yarn test) were run locally and passed
• Visual Regression Tests (yarn visual-regression) were run locally and passed
• Linting (npm lint) was run locally and passed

Pull request type

Please check the type of change your PR introduces:

• Bug fix
• Feature
• Refactoring (no functional changes, no API changes)
• Build related changes
• Documentation content changes
• Other (please describe):

What is the new behavior?

• Implemented keyboard accessibility of the <ix-burger-menu> component by inserting a <button> element.
• Implemented new property 'ixAriaLabel' for the <ix-burger-menu> component, which is required as an accessibility label for the respective icon button.
• Set the ARIA-attributes 'aria-label' and 'aria-pressed' for the <ix-burger-menu> component.
• Adjusted focus frame settings of the <ix-burger-menu> component via CSS.

Does this introduce a breaking change?

• Yes
• No

Testing

Other information

[nuke-ellington]

Something like this should do the trick

[nuke-ellington]

lgtm