Add MixedBooleanOperatorSniff

[TimWolla]

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Hi :wave:

This is my first contribution to PHP CodeSniffer. This PR adds a sniff that detects mixed boolean operators (&&, ||) within a single expression without making precedence clear using parentheses.

I hope I did not miss an existing sniff that already does what I want this sniff to do. Please also let me know if I violated any written or unwritten rules regarding the namespace structure, code formatting, commit messages, etc or if you think I missed some edge case within my unit test.

[TimWolla]

I was going through the list of my open PRs:

I'm seeing that this PR was assigned to a project / to the idea bank. How will this process further? I don't plan on making any more changes, unless you see issues during review.

[gsherwood]

Sorry this hasn't moved. I've been prioritising bug fixes and support for newer PHP versions so I haven't looked into this yet.

[TimWolla]

Understood, thank you for the update.

[TimWolla]

Hi! It's been another 6 months. I've just rebased my branch on the latest master.

Do you have some cycles to take a look at this?