kanata: fn in defsrc block

[ssbb]

fn is a valid keycode in defsrc block for kanata layouts.

Right now it raises because it's not defined in kanata json file.

[caksoylar]

What does fn correspond to in keycodes?

[ssbb]

Kanata maps it here: https://github.com/jtroo/kanata/blob/c676c4a594c738cef3dbfc59a5166cad72aec2a2/parser/src/keys/mod.rs#L326

On some keyboards (like Apple) Fn key processed on OS side so can be used for Kanata-based layer switching as well. On ZMK it maps to GLOBE key I believe.