This snippet goes inside the `rules` array of ~/.config/karabiner/karabiner.json

karabiner_caps-to-esc-ctrl.json

{
    "manipulators": [
        {
            "description": "Change caps_lock to control when used as modifier, escape when used alone (~/.config/karabiner/karabiner.json)",
            "from": {
                "key_code": "caps_lock",
                "modifiers": {
                    "optional": [
                        "any"
                    ]
                }
            },
            "to": [
                {
                    "key_code": "left_control"
                }
            ],
            "to_if_alone": [
                {
                    "key_code": "escape"
                }
            ],
            "type": "basic"
        }
    ]
}