Improve link checker

Was ignoring a whole class of potential failures (caught by
https://docs-dev.raku.org/error-report)

Author: coke

xt/rakudoc-l.rakutest

@@ -85,6 +85,13 @@ sub is-valid-link($links) {
             my $ok = ?($path.IO.d or ($path ~ '.rakudoc').IO.e);
             ok $ok, "$link exists (primary)";
             %bad-links{$link}++ unless $ok;
+        } else {
+            my @valid-protocols = <http https irc>;
+            next if $link.starts-with: '#'; # don't check in-page anchors
+            next if $link.starts-with(any(@valid-protocols) ~ '://');
+
+            %bad-links{$link}++;
+            flunk("relative link or bad protocol: $link");
         }
     }
 }