LibFileSystem+Userland: Rename size() to size_from_stat()

This reflects what the functions does more accurately, and allows for
adding functions to get sizes through other methods.

This also corrects the return type of said function, as size_t may only
hold sizes up to 4GB on 32-bit platforms.

Userland/Services/WebServer/Client.cpp

@@ -170,7 +170,7 @@ ErrorOr<bool> Client::handle_request(HTTP::HttpRequest const& request)
 
     auto const info = ContentInfo {
         .type = TRY(String::from_utf8(Core::guess_mime_type_based_on_filename(real_path.bytes_as_string_view()))),
-        .length = TRY(FileSystem::size(real_path.bytes_as_string_view()))
+        .length = static_cast<u64>(TRY(FileSystem::size_from_stat(real_path.bytes_as_string_view())))
     };
     TRY(send_response(*stream, request, move(info)));
     return true;

Userland/Services/WebServer/Client.h

@@ -28,7 +28,7 @@ private:
 
     struct ContentInfo {
         String type;
-        size_t length {};
+        u64 length {};
     };
 
     ErrorOr<void, WrappedError> on_ready_to_read();