Update Perl6 syntax highlighting to allow for kebab-case in addition to snake_case

[aeschli]

From @WildYorkies on February 13, 2017 21:22

Steps to Reproduce:

1. Make a file called test.pl6 or test.pm6
2. Type the following in:

sub print-board() # Wrong

sub thing-for-that() # Wrong

3. See that the words print and for are highlighted as if they are keywords.

4. Change it to snake case and notice that it is like a normal variable again:

sub print_board() # Correct

sub thing_for_that() # Correct

Solution:

Update this file with the kebab case addition.

Copied from original issue: microsoft/vscode#20563

[aeschli]

It's a problem of https://github.com/textmate/perl.tmbundle/blob/master/Syntaxes/Perl%206.tmLanguage that we use.