Replaces []Format with a Group type.
A bit more type safe.
Breaking change for RegisterFormat, now takes a first argument that is a "single" format group.
Lots of naming cleanup.
This is also preparation for decode group argument which will enable doing intresting
probing, ex a format decoder could know it's decode as part of probe group (html could
be probed possibly), or have "arg probe" group for decoder who inspect args to know
if they should probe (-d /path/to/schema etc) to enable nice CLI-ergonomics.
Move registry to interp and add support for functions and filesystems.
This will be used later for allow formats to add own functions and fq code.
Add gojqextra function helpers to have more comfortable API to add functions.
Takes care of argument type casting and JQValue:s and some more things.
Refactor interp package to use new function helper and registry. Probably
fixes a bunch of JQValue bugs and other type errors.
Refactor out some mpeg nal things to mpeg format.
Refactor interp jq code into display.q and init.jq.
Remove undocumented aes_ctr funciton, was a test. Hopefully will add more crypto things laster.
