Add sniff to ban heredocs.

[jaymcp]

Per internal discussion, we have decided to disallow the use of heredocs (nowdocs are okay). This PR introduces a Sniff to that effect.

Types of changes (if applicable):

• Bug fix (non-breaking change which fixes an issue).
• New feature (non-breaking change which adds functionality).
• Breaking change (fix or feature that would cause existing functionality to not work as expected).

Checklist (if applicable):

• All checks pass when running `composer run all-checks.