fix: Improve the keyboard and screenreader accessibility of the backpack

[gonfunko]

The basics

• I validated my changes

The details

Resolves

Fixes #2711

Proposed Changes

This PR improves the keyboard navigability and screenreader accessibility of the backpack plugin. Now,

• The backpack can be tabbed to
• The contextual menu can be shown via the keyboard
• The backpack has a button role and appropriate ARIA label depending on its current state
• When focused, the backpack has a focus ring and darkens as it does on hover
• When opened, the first block in the flyout is automatically focused

[mikeharv]

Worth explaining magic numbers (or using constants)?