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.

Meta/gn/secondary/AK/BUILD.gn

@@ -291,6 +291,7 @@ write_cmake_config("ak_debug_gen") {
     "IMAP_PARSER_DEBUG=",
     "ITEM_RECTS_DEBUG=",
     "JOB_DEBUG=",
+    "JBIG2_DEBUG=",
     "JPEG_DEBUG=",
     "JS_BYTECODE_DEBUG=",
     "JS_MODULE_DEBUG=",

Meta/gn/secondary/Userland/Libraries/LibGfx/BUILD.gn

@@ -74,6 +74,7 @@ shared_library("LibGfx") {
     "ImageFormats/ISOBMFF/Boxes.cpp",
     "ImageFormats/ISOBMFF/Reader.cpp",
     "ImageFormats/ImageDecoder.cpp",
+    "ImageFormats/JBIG2Loader.cpp",
     "ImageFormats/JPEGLoader.cpp",
     "ImageFormats/JPEGWriter.cpp",
     "ImageFormats/JPEGXLLoader.cpp",