fix: disabled menu items should be focusable

[scomea]

Description

The menu spec specifies that "disabled menu items are focusable but cannot be activated", but the implementation did not allow focus on disabled items. This changes the implementation to match the spec.

Motivation & context

• noticed behavior did not match spec. Focusability allows discovery of disabled items for users of assistive technology.

Issue type checklist

• Chore: A change that does not impact distributed packages.
• Bug fix: A change that fixes an issue, link to the issue above.
• New feature: A change that adds functionality.

Is this a breaking change?

• This change causes current functionality to break.

Adding or modifying component(s) in @microsoft/fast-components checklist

• I have added a new component
• I have modified an existing component
• I have updated the definition file
• I have updated the configuration file

Process & policy checklist

• I have added tests for my changes.
• I have tested my changes.
• I have updated the project documentation to reflect my changes.
• I have read the CONTRIBUTING documentation and followed the standards for this project.