Fix stringable sniff to account for namespaces

[jaymcp]

Description

Fixes the fixer within the BigBite.Classes.Stringable Sniff to prepend the interface name with a backslash, to ensure it references the global namespace in situations where the class being modified is namespaced.

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.