Function subscribe

src/fitty.js:201–205

(f)

    // add a new fitty, does not redraw said fitty
    const subscribe = (f) => () => {
        if (f.active) return;
        f.active = true;
        requestRedraw();
    };

    // remove an existing fitty
    const unsubscribe = (f) => () => (f.active = false);