Please mark Builder methods @pure to ease understanding that it's immutable

[b1rdex]

After upgrading to v5, I spent some time figuring out why the code broke, but this wasn't detected by PHPStan or any other tool.

After some tests, I found out that the reason is making the Builder immutable (#979) in the version.

Please consider marking the methods @pure, so SCA can report that 'the result is not used'.

Rule check example: https://phpstan.org/r/f638ba16-0ab3-44e2-becd-2927e99ada09

[Slamdunk]

Thank you for your rightful complaint: could you please send a PR to fix this?

[b1rdex]

@Slamdunk done: #1076