Add support for chords to mapper tool

It would be great if we could also display chords in the mapper tool, similarly to VSCode.

I dug into this a bit and it's more complicated than I expected. If you just have a timer and track keydown events, you get chords like Meta Meta+b because the keydown for Meta is different from the keydown for b. It looks like this is solved in the main code with the 'radix trie' logic, but this seems pretty closely tied to the hotkeys that are configured on the element, and we can't configure _all_ possible hotkeys onto the mapper tool.

首先跟踪 issue 中描述的 mapper tool 和主代码的 radix trie 逻辑，然后比较 VSCode 如何显示 chords。确定当 mapper 无法配置所有可能的 hotkeys 时，应如何对 modifier 和 keydown 事件进行分组；当 mapper 显示 chords 且没有重复的 modifier 条目时，即表示完成。

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