Forbid @template annotation on closures

[weirdan]

Hack doesn't support templated closures, and I don't think there's much value in Psalm supporting them either. Opens up a whole can of worms.

Originally posted by @muglug in #5471 (comment)

It's currently accepted, but does not produce a useful generic: https://psalm.dev/r/93bb04856b. It would be better to reject it outright.

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/93bb04856b

<?php
$f = 
/**
 * @template T
 * @param T $t
 * @return T
 */
 fn($t) => $t;

$f(42);

Psalm output (using commit 2edf613):

ERROR: InvalidArgument - 10:4 - Argument 1 expects T:fn-/var/www/vhosts/psalm.dev/httpdocs/src/somefile.php:8:64:-:closure as mixed, 42 provided

[weirdan]

Expected: InvalidDocblock: Templated closures are not supported