Is there a way to use alphabet keys as a modifier

Let's say I want the "a" key to be a modifier, so when I hold "a" and hit "h", skhd will simulate "left". I'd like the "a" key to still send "a" when tapped, similar to [QMK's mod-tap](https://docs.qmk.fm/#/mod_tap).

Something like this: `a - h : skhd -k "left"`

I want to try this with both alphabet keys and F-keys if possible.

The issue names no files or tests. Start by tracing skhd's existing hotkey parsing and macOS key-event handling, then compare the requested behavior with QMK's mod-tap model. Done means a held alphabet or F-key can participate in a combination such as a+h while a tap still emits a.

Written by the indexing model from the issue text.