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/CMakeLists.txt

@@ -47,6 +47,7 @@ set(SOURCES
     ImageFormats/ImageDecoder.cpp
     ImageFormats/ISOBMFF/Boxes.cpp
     ImageFormats/ISOBMFF/Reader.cpp
+    ImageFormats/JBIG2Loader.cpp
     ImageFormats/JPEGLoader.cpp
     ImageFormats/JPEGXLLoader.cpp
     ImageFormats/JPEGWriter.cpp