LibURL+Everywhere: Only percent decode URL paths when actually needed

Web specs do not return through javascript percent decoded URL path
components - but we were doing this in a number of places due to the
default behaviour of URL::serialize_path.

Since percent encoded URL paths may not contain valid UTF-8 - this was
resulting in us crashing in these places.

For example - on an HTMLAnchorElement when retrieving the pathname for
the URL of:

http://ladybird.org/foo%C2%91%91

To fix this make the URL class only return the percent encoded
serialized path, matching the URL spec. When the decoded path is
required instead explicitly call URL::percent_decode.

This fixes a crash running WPT URL tests for the anchor element on:

https://wpt.live/url/a-element.html

Tests/LibWeb/Text/input/HTML/anchor-element-url-invalid-unicode-pathname.html

@@ -0,0 +1,8 @@
+<a id="anchor" href="http://ladybird.org/foo%C2%91%91"></a>
+<script src="../include.js"></script>
+<script>
+    test(() => {
+        const anchorElement = document.getElementById('anchor');
+        println(anchorElement.pathname);
+    })
+</script>