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

从 index.js 的第 228-236 行开始，这里会根据键盘事件匹配热键，并在使用非 en-US 布局（例如 ru-RU）的 Chrome 或 Vivaldi 中重现该问题。跟踪当前的匹配行为，并验证每个已记录的热键无论键盘布局如何都会触发相同的命令。

由索引模型根据 Issue 内容生成。