patterns: add DICOM

DICOM files specify a "transfer syntax" which defines the image
compression format, the encoding rules for the entire file, and any
special-case encoding rules for particular fields.  Many transfer syntaxes
have common design features, but a transfer syntax is allowed to use
completely custom encoding rules, and need not be publicly documented.

Explicitly reject transfer syntaxes we know we don't support: implicit VR
(uncommon), explicit VR big-endian (deprecated), deflate-compressed.
In all other cases, follow some general rules and hope it works out.

The large lookup functions were generated by a script, but I haven't
included the code here, since its input is hand-massaged data copied from
HTML tables.

Add a JPEG WSI test case from the WG26 2021 hackathon (CC0 license) and a
synthetic LE Explicit VLP case produced with img2dcm.