Mixed should report error in encapsed like it does in concat

[kkmuffme]

https://psalm.dev/r/bec8508d02

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/bec8508d02

<?php

/**
 * @var mixed $y
 */
$x = "hello" . $y . "world";

$z = "hello {$y} world";

Psalm output (using commit 08afc45):

INFO: MixedOperand - 6:16 - Right operand cannot be mixed

INFO: UnusedVariable - 6:1 - $x is never referenced or the value is not used

INFO: UnusedVariable - 8:1 - $z is never referenced or the value is not used