AK: Decode data URLs to separate class (and parse like every other URL)

Parsing 'data:' URLs took it's own route. It never set standard URL
fields like path, query or fragment (except for scheme) and instead
gave us separate methods called `data_payload()`, `data_mime_type()`,
and `data_payload_is_base64()`.

Because parsing 'data:' didn't use standard fields, running the
following JS code:

    new URL('#a', 'data:text/plain,hello').toString()

not only cleared the path as URLParser doesn't check for data from
data_payload() function (making the result be 'data:#a'), but it also
crashes the program because we forbid having an empty MIME type when we
serialize to string.

With this change, 'data:' URLs will be parsed like every other URLs.
To decode the 'data:' URL contents, one needs to call process_data_url()
on a URL, which will return a struct containing MIME type with already
decoded data! :^)

AK/URLParser.h

@@ -63,9 +63,6 @@ public:
 
     // https://url.spec.whatwg.org/#concept-host-serializer
     static ErrorOr<String> serialize_host(URL::Host const&);
-
-private:
-    static Optional<URL> parse_data_url(StringView raw_input);
 };
 
 #undef ENUMERATE_STATES