add syntax for Debian control files DCF

[aronatkins]

Debian control files (DCF) are used in a variety of contexts beyond describing Debian package files. The R language, for example, uses DCF to describe its packages.

https://www.debian.org/doc/debian-policy/ch-controlfields.html
https://cran.r-project.org/doc/manuals/r-release/R-exts.html#The-DESCRIPTION-file
https://github.com/KDE/syntax-highlighting/blob/master/data/syntax/debiancontrol.xml

This is not an urgent issue, as I can add this XML syntax definition locally.