filters: rewrite colorize in c

Since its introduction, we had multiple issues with the colorize awk
script with regard to non-GNU awk compatibility.

Also, this script is standalone and the color theme must be hard coded
into it. Reading from an external configuration file (aerc's styleset)
from a non-GNU awk is close to impossible (and even far from trivial
with GNU awk).

Rewrite the builtin colorize filter in C to allow getting the color
theme from aerc's active styleset. The theme is configured using the
existing styleset syntax and attributes under a separate [viewer]
section (see examples and man page).

Export the active styleset file path to AERC_STYLESET env var when
invoking the filter command so that colorize can access it and use it.

I have tested compilation (with clang-analyzer and gcc -fanalyzer) and
basic operation on FreeBSD, Fedora (glibc) and Alpine (muslibc). More
tests would probably be required on MacOSX and older Linux distros.
I also added test vectors to give some confidence that this works as
expected. The execution with these vectors passed valgrind
--leak-check=full without errors.

NB: the default theme has changed to be more minimal. Sample stylesets
have more colorful examples. The awk -v theme=xxx option is no longer
supported.

  usage: colorize [-h] [-s FILE] [-f FILE]

  options:
    -h       show this help message
    -s FILE  use styleset file (default $AERC_STYLESET)
    -f FILE  read from filename (default stdin)

Signed-off-by: Robin Jarry <robin@jarry.cc>
Tested-by: Bence Ferdinandy <bence@ferdinandy.com>
Acked-by: Moritz Poldrack <moritz@poldrack.dev>

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