plguins/lsp: add perlpls language server

[GeoffreyFrogeye]

I haven't been able to successfully run the test suite, but I suspect it's not related to my changes (logs, seems like cache.nixos.org went down at some point?). It's currently trying to run again but since it takes a while and I might forget I wanted to at least post the MR. Oh well CI passed the tests, nevermind.

If you want to test manually without learning Perl (wouldn't recommend 🙈), create a .pl file with the following content:

use strict;
printf "$var\n";

It should complain about $var not being declared.