Additions to the Perl family of languages (#4066)

* Mainly fixing problems with Perl heuristics

And also adding a little bit of text to the README file to help with local use and test.

* Adds new sample

* Adds a couple of samples more, not represented before

* Moves installation intructions to CONTRIBUTING.md

Refs #2309 and also changes github.com to an uniform capitalization.

* Correcting error. Great job, CI

* Moving another file

* Adds samples and new checks for perl/perl6

* Stupid mistake

* Changing regex for perl5 vs perl6

Initial suggestion by @pchaigno, slightly changed to eliminate false positives such as "classes" or "modules" at the beginning of a line in the =pod

BTW, it would be interesting to just eliminate these areas for language detection.

* Eliminates Rexfile from Perl6

And adds .pod6

* Followup to #2709

I just found I had this sitting here, so I might as well follow
instructions to fix it.

* Adds example for pod6

* Eliminates .pod because it's its own language

* Removes bad directory

* Reverting changes that were already there

* Restored CONTRIBUTING.md from head

I see installation of cmake is advised in README.md

* Eliminates `.pod6`

To leave way for #3366 or succeeding PRs.

* Removed by request, since we're no longer adding this extension

* Sorting by alphabetical order filenames

* Moved from sample to test fixtures

test/fixtures/Perl 6/chromosome.pl

@@ -0,0 +1,9 @@
+class Chromosome {
+  has Seq $.chromosome is rw;
+  has $.fitness is rw;
+
+}
+
+my $len = 32;
+my $this-chromosome = Chromosome.new( chromosome => map( { rand >= 0.5 ?? True !! False }, 1..$len )  );
+say $this-chromosome.chromosome();