Handle relative image URLs in the Pagefind UI

[bglw]

Discussed in https://github.com/CloudCannon/pagefind/discussions/529

^{Originally posted by **kenmorse** December 19, 2023} So for a page bundle, you might have something like this in your source:  instead of a full path to the image like this:  This seems to be a problem for Pagefind though, as the search results panel won't have a full path to the image.

[bglw]

Workaround from discussion:

new PagefindUI({
    element: "#search",
    showSubResults: true,
    showImages: true,
    processResult: function (result) {
        if (result?.meta?.image) {
            let resultBase = new URL(result.url, window.location);
            let remappedImage = new URL(result.meta.image, resultBase);
            if (remappedImage.hostname !== window.location.hostname) {
                result.meta.image = remappedImage.toString();
            } else {
                result.meta.image = remappedImage.pathname;
            }
        }
    }
});

This can pretty much be rolled into the Default (and Modular) UI handling verbatim. I'm wary of adding it any earlier in the flow, as someone might be relying on the indexing and JS API to not mutate metadata.