Javascript – Common code visible at root scope in Svelte

I integrated the Google Sign In button (https://developers.google.com/identity/gsi/web/guides/personalized-button) in a Svelte application v.3.54.0.
The button requires a function in the global scope as a callback (data-callback attribute), e.g.:

    <div
        id="g_id_onload"
        data-client_id="<id>.apps.googleusercontent.com"
        data-context="signin"
        data-ux_mode="popup"
        data-callback="handleCredentialResponse"
        data-auto_prompt="false"
    />

    <div
        class="g_id_signin"
        data-type="standard"
        data-shape="rectangular"
        data-theme="filled_blue"
        data-text="signin_with"
        data-size="large"
        data-logo_alignment="left"
    />
            

I would like the handleCredentialResponse function to reside in a source file other than app.html, because I need to use other imported modules in the implementation, e.g.:

    function handleCredentialResponse(response) {
        
        localStorage.setItem("googleCredentials", JSON.stringify(response));
        let jwt = jwt_decode(JSON.parse(response).credential);
        localStorage.setItem('decodedJwt', JSON.stringify(jwt));
        window.location.reload();
    } 

Here I use jwt_decode which is imported from jwt-decode.
If I include handleCredentialResponse in a script tag in app.html, I get an error because jwt_decode is not defined at this level. I would have to include jwt_decode via another script tag, which I rather avoid.
On the other hand, if I write the function in a Svelte component (even with context="module"), the Google Button complains because the callback is not visible to it.
Currently I use a workaroud where I have part of the logic in app.html and part in the +layout.svelte onMount, but I really do not like it.
What is the Svelte way to deal with this need? Or, in other words, is there a place in a svelte application where I can write common code that is also visible at the root scope (and, possibly, that gets executed only once at the first loading)?