Add support for OAuth 2.0 authorization code flow without requiring a custom button.

[astarrh]

This PR introduces an authMode prop to enable support for the OAuth 2.0 authorization code flow in the GoogleLogin component in addition to the existing credential-based flow. This is accomplished by using the GIS button style and replacing the click listener with logic from google.accounts.oauth2.initCodeClient.:

if (button) {
  window.google.accounts.id.renderButton(button, buttonConfig);
  const newButton = button.cloneNode(true);
  button.parentNode!.replaceChild(newButton, button);
  newButton.addEventListener("click", () => {
    oauthClient.requestCode(); // Custom behavior
  });
}

Default behavior remains unchanged, but developers may toggle the new behavior with the authMode prop:

<script setup>
const callback = (response) => {
  // This callback will be triggered when the user logs in
  // and receives an authorization code
  console.log("Authorization code:", response.code);
};
</script>

<template>
  <GoogleLogin :callback="callback" :authMode="'code'" />
</template>

Default behavior may be set explicitly with authMode="'credential'". This module doesn't contain a testing package, but on my system it is working as expected and is backwards compatible.

Key Changes in the PR:
1. Added authMode: "code" to support google.accounts.oauth2.initCodeClient.
2. Updated the GoogleLogin component to dynamically render buttons based on the selected flow.
3. Ensured backward compatibility with existing credential behavior.
4. Added documentation and examples for the new authMode prop.

Closes #71