LibGfx/ILBM: Add an IFF-ILBM decoder :)

IFF was a generic container fileformat that was popular on the Amiga
since it was the only file format supported by Deluxe Paint.

ILBM is an image format popular in the late eighties/nineties
that uses the IFF container.

This is a very first version of the decoder that only supports
(byterun) compressed files with bpp <= 8.

Only the minimal chunks are decoded: CMAP, BODY, BMHD.

I am planning to add support for the following variants:

- EHB (32 colours + lighter 32 colours)
- HAM6 / HAM8 (special mode that allowed to display the whole Amiga
4096 colours / 262 144 colours palette)
- TrueColor (24bit)

Things that could be fun to do:

- Still images could be animated using color cycle information

Userland/Libraries/LibCore/MimeData.cpp

@@ -132,6 +132,7 @@ static Array const s_registered_mime_type = {
     MimeType { .name = "image/vnd.ms-dds"sv, .common_extensions = { ".dds"sv }, .description = "DDS image data"sv, .magic_bytes = Vector<u8> { 'D', 'D', 'S', ' ' } },
     MimeType { .name = "image/webp"sv, .common_extensions = { ".webp"sv }, .description = "WebP image data"sv, .magic_bytes = Vector<u8> { 'W', 'E', 'B', 'P' }, .offset = 8 },
     MimeType { .name = "image/x-icon"sv, .common_extensions = { ".ico"sv }, .description = "ICO image data"sv },
+    MimeType { .name = "image/x-ilbm"sv, .common_extensions = { ".iff"sv }, .description = "Interleaved bitmap image data"sv, .magic_bytes = Vector<u8> { 0x46, 0x4F, 0x52, 0x4F } },
     MimeType { .name = "image/x-portable-bitmap"sv, .common_extensions = { ".pbm"sv }, .description = "PBM image data"sv, .magic_bytes = Vector<u8> { 0x50, 0x31, 0x0A } },
     MimeType { .name = "image/x-portable-graymap"sv, .common_extensions = { ".pgm"sv }, .description = "PGM image data"sv, .magic_bytes = Vector<u8> { 0x50, 0x32, 0x0A } },
     MimeType { .name = "image/x-portable-pixmap"sv, .common_extensions = { ".ppm"sv }, .description = "PPM image data"sv, .magic_bytes = Vector<u8> { 0x50, 0x33, 0x0A } },