LibGfx+Ladybird+Userland: Don't sniff for TGA images with only raw bytes

Because TGA images don't have magic bytes as a signature to be detected,
instead assume a sequence of ReadonlyBytes is a possible TGA image only
if we are given a path so we could check the extension of the file and
see if it's a TGA image.

When we know the path of the file being loaded, we will try to first
check its extension, and only if there's no match to a known decoder,
based on simple extension lookup, then we would probe for other formats
as usual with the normal sniffing method.

Userland/Libraries/LibImageDecoderClient/Client.h

@@ -31,6 +31,7 @@ class Client final
 
 public:
     Optional<DecodedImage> decode_image(ReadonlyBytes);
+    Optional<DecodedImage> decode_image_with_known_path(DeprecatedString const& path, ReadonlyBytes);
 
     Function<void()> on_death;