fix: ensure Escape key reliably closes popups

[nfreear]

Problem

Currently (Leaflet 1.9.4), the Escape key seems to close pop-ups when the map container has keyboard focus, but not when pop-ups or other elements within the map have keyboard focus.

• Related core Leaflet issue: Closing Popup by pressing Escape doesn't work in all cases Leaflet/Leaflet#6551 (2019),
• Relates to: Focus should be managed between markers and popups #3,
• WCAG 2.2: Success Criteria 2.1.1 Keyboard.

Needs investigation.