Exposing saveCurrentPageToHistory via js API

When working with boosted error responses like e.g. in Laravel it can occur that there is a custom error page with custom head tags which have nothing to do with my htmx-powered application. This site is totally different and it is not enough to "just" swap the body.
I solved this using the following code:
```js
document.body.addEventListener('htmx:beforeOnLoad', function (evt) {
if (evt.detail.xhr.status >= 400) {
htmx.saveCurrentPageToHistory()
document.location = evt.detail.xhr.responseURL;
let iframe = document.createElement('iframe');
document.getElementById('htmx-error-modal-content').appendChild(iframe);
iframe.src = 'about:blank';
iframe.contentWindow.document.open();
iframe.contentWindow.document.write(evt.detail.xhr.responseText);
iframe.contentWindow.document.close();
document.getElementById('htmx-error-modal-backdrop').style.display = 'block';
}
});
```

Without the exported `htmx.saveCurrentPageToHistory()` the forward/backward buttons would not work properly. Therefore, I propose to add it to the exported functions.

Thank you in advance