use perl-navigator as default language server

[Freed-Wu]

Currently, coc-perl support 2 LS: perl-languageserver and perlnavigator. coc-perl force user to choose one in config.

I notice perlnavigator is written in js, which means it should be easy as a pacakage's dependency by coc-perl. We can let it as default LS to save user's time for their first trial of coc-perl. If they are not statisified, they can install perl-languageserver by themselves.

Just my 2c. Thanks!