Hotkeys don't work on Windows with non en-us keyboard layout

Current way of matching hotkey with `event.key` property leads to broken hotkeys on Windows (Chrome, Vivaldi at least) as, for example with ru-RU locale on keybord hitting cmd+b will send event with `key` property set to cyrillic "и" and not "b".

First it was found in https://github.com/umputun/remark/issues/404

Images provided by @Andrew-Shtein prove difference in `key` prop

Issue located here:
https://github.com/github/markdown-toolbar-element/blob/dd0ffba8d9ad9d64471d3be41b3b7e1b77aa3545/index.js#L228-L236

Start in index.js at lines 228-236, where hotkeys are matched from the keyboard event, and reproduce the issue in Chrome or Vivaldi with a non-en-US layout such as ru-RU. Trace the current matching behavior and verify that each documented hotkey triggers the same command regardless of keyboard layout.

Written by the indexing model from the issue text.