LibWeb: Move document loading into separate file

In upcoming navigables refactor new function that uses fetch
instead of ResourceLoader to load document content is going to be
introduced:
https://html.spec.whatwg.org/multipage/browsing-the-web.html#create-navigation-params-by-fetching

`parse_document()` need to be separated from FrameLoader to reuse
code responsible for parsing HTTP response into DOM document.

Userland/Libraries/LibWeb/Loader/FrameLoader.h

@@ -45,7 +45,6 @@ private:
 
     void load_error_page(const AK::URL& failed_url, DeprecatedString const& error_message);
     void load_favicon(RefPtr<Gfx::Bitmap> bitmap = nullptr);
-    bool parse_document(DOM::Document&, ByteBuffer const& data);
 
     JS::NonnullGCPtr<HTML::BrowsingContext> m_browsing_context;
     size_t m_redirects_count { 0 };