Add syntax highlighting for Syntax::Keyword::MultiSub

[bscan]

This would be defined here https://github.com/bscan/PerlNavigator/blob/main/syntaxes/perl-scopes.json, and pointing to another .json file. The goal would be highlighting the word multi here:

use Syntax::Keyword::MultiSub;
use experimental 'signatures';
 
multi sub max()          { return undef; }
multi sub max($x)        { return $x; }
multi sub max($x, @more) { my $y = max(@more);
                           return $x > $y ? $x : $y; }
 
say max(1, 2, 15, 3, 4);  # prints 15