Implement Sniff to ensure a blank line after an object's opening brace

[jaymcp]

Description

Our coding standards require a blank line between an object (class, enum, interface, trait) definition's opening brace and first content, but this is not enforced by our PHPCS ruleset. This PR addresses that by introducing a new Sniff.

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.