feat: add minimal support for Debian pre-dependencies

[jeremy-gill]

Basic support for adding 'Pre-Depends:' to Debian packages added.

[jeremy-gill]

@caarlos0, I've moved the 'predepends' tag from the platform-independent area to a Debian-specific location. I've also tweaked deb/deb_test.go to provide some minimal testing of the new tag. Finally, I updated the www/docs/configuration.md to explain the use of the new tag. Is this the change you were looking for?

[codecov[bot]]

Codecov Report

Merging #696 (94b5690) into main (24a43c5) will increase coverage by 0.01%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #696      +/-   ##
==========================================
+ Coverage   75.02%   75.03%   +0.01%     
==========================================
  Files          10       10              
  Lines        2406     2407       +1     
==========================================
+ Hits         1805     1806       +1     
  Misses        425      425              
  Partials      176      176              

Files Changed	Coverage Δ
deb/deb.go	72.42% <ø> (ø)
nfpm.go	85.06% <100.00%> (+0.06%)	:arrow_up:

[caarlos0]

thank you, I'll add an integration test too.