Fix top-level burger menu keyboard navigation

[nwmac]

Summary

Fixes #12769

Occurred changes and/or fixed issues

This PR adds keyboard navigation support to the top-left app bar hamburger menu.

Areas or cases that should be tested

Load the UI, press the Tab key - you will see focus indication on the menu icon.

You can now press SPACE or ENTER to expand/collapse the button.

This also adds the aria-label and role attributes to the button.

Screenshot/Video

Checklist

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[aalves08]

LGTM Neil. Let's get this first one in, so that we can build on top of this app-wide focus style 🙏