AK+Everywhere: Change URL::path() to serialize_path()

This now defaults to serializing the path with percent decoded segments
(which is what all callers expect), but has an option not to. This fixes
`file://` URLs with spaces in their paths.

The name has been changed to serialize_path() path to make it more clear
that this method will generate a new string each call (except for the
cannot_be_a_base_url() case). A few callers have then been updated to
avoid repeatedly calling this function.

Userland/Libraries/LibWeb/HTML/BrowsingContext.cpp

@@ -39,7 +39,7 @@ static bool url_matches_about_blank(AK::URL const& url)
 {
     // A URL matches about:blank if its scheme is "about", its path contains a single string "blank", its username and password are the empty string, and its host is null.
     return url.scheme() == "about"sv
-        && url.path() == "blank"sv
+        && url.serialize_path() == "blank"sv
         && url.username().is_empty()
         && url.password().is_empty()
         && url.host().is_null();