Include .t files in Perl::LanguageServer fileFilter

Serena's file-to-language router already maps `.t` to the Perl LS
(`Language.PERL` in `solidlsp/ls_config.py`), but the
Perl::LanguageServer-side `fileFilter` was hardcoded to
`[".pm", ".pl"]`. As a result Perl::LanguageServer skipped `.t`
files entirely: symbol queries on a `.t` returned empty, and
references to `.pm`/`.pl` subs missed any callers defined in
`.t` files.

Add `.t` to the filter, deduplicate the two `fileFilter` literals
into a `_FILE_FILTER` class constant, and note the sync requirement
with `Language.PERL.get_source_fn_matcher()` in a comment. Cover
the behavior with a new cross-file references test plus a `helper.t`
fixture; the test fails on unpatched `main` and passes on this
branch.

test/resources/repos/perl/test_repo/helper.t

@@ -0,0 +1,12 @@
+#!/usr/bin/env perl
+
+use lib '.';
+use strict;
+use warnings;
+
+use Test::More tests => 1;
+
+require helper;
+
+helper_function();
+ok(1, 'helper_function callable from .t file');