Forbid assertions based on non-pure expressions

[morozov]

Currently, such assertions are not reported as an issue: https://psalm.dev/r/d5d33b6c18 but in practice, they are (see doctrine/dbal#4359).

Given that the behavior of the assert() function is different depending on the development/production settings, the side effects within the assertions cannot be relied upon and shouldn't be allowed.

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/d5d33b6c18

<?php

function nonPure(): bool {
    echo 'SIDE EFFECT';

    return true;
}

assert(nonPure());

Psalm output (using commit fd43ba7):

No issues!