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.

Start by tracing the mapper tool and the main code's radix trie logic described in the issue, then compare how VSCode displays chords. Determine how modifier and keydown events should be grouped when all possible hotkeys cannot be configured on the mapper; done means the mapper displays chords without duplicate modifier entries.

索引モデルが issue の本文から書いたものです。