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.

consts/dicom.pat was 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.

README.md

@@ -118,6 +118,7 @@ Everything will immediately show up in ImHex's Content Store and gets bundled wi
 
 | Name | Path | Description |
 |------|------|-------------|
+| libconsts | [`includes/consts/*`](includes/consts) | Constant values for use in patterns |
 | libhex | [`includes/hex/*`](includes/hex) | Functions to interact with ImHex |
 | libstd | [`includes/std/*`](includes/std) | Pattern Language Standard Library |
 | libtype | [`includes/type/*`](includes/type) | Various custom types with special formatters |