LibGfx: Add the start of a JBIG2 loader

JBIG2 is infamous for two things:

1. It's used in xerox scanners were it falsifies scanned numbers:

https://www.dkriesel.com/en/blog/2013/0802_xerox-workcentres_are_switching_written_numbers_when_scanning

2. It was allegedly used in an iOS zero day, in a very cool way:

https://googleprojectzero.blogspot.com/2021/12/a-deep-dive-into-nso-zero-click.html

Needless to say, we need support for it in Serenity. (...because it's
used in PDF files.)

This adds all the scaffolding, but no actual implementation yet.

It's enough for `file` to print the mime type of .jb2 files, but `image`
can't do anything with the files yet.

Userland/Libraries/LibGfx/ImageFormats/JBIG2Loader.cpp

@@ -0,0 +1,26 @@
+/*
+ * Copyright (c) 2024, Nico Weber <thakis@chromium.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibGfx/ImageFormats/JBIG2Loader.h>
+
+// Spec: ITU-T_T_88__08_2018.pdf in the zip file here:
+// https://www.itu.int/rec/T-REC-T.88-201808-I
+
+namespace Gfx {
+
+bool JBIG2ImageDecoderPlugin::sniff(ReadonlyBytes data)
+{
+    // JBIG2 spec, Annex D, D.4.1 ID string
+    u8 id_string[] = { 0x97, 0x4A, 0x42, 0x32, 0x0D, 0x0A, 0x1A, 0x0A };
+    return data.starts_with(id_string);
+}
+
+ErrorOr<NonnullOwnPtr<ImageDecoderPlugin>> JBIG2ImageDecoderPlugin::create(ReadonlyBytes)
+{
+    return Error::from_string_view("FIXME: Draw the rest of the owl"sv);
+}
+
+}