Squiz/FunctionDeclarationArgumentSpacing: use placeholders in error messages

[ntdiary]

Description

Replace string concatenation with placeholders to follow the best practice.

Suggested changelog entry

Changed

• Squiz.Functions.FunctionDeclarationArgumentSpacingSniff: improvements to error messages
  • SpaceBeforeEquals error message now exposes 3 data values (previously 2).
  • SpaceAfterEquals error message now exposes 3 data values (previously 2).

Related issues/external references

related to #1240

Types of changes

• 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 change)
  • This change is only breaking for integrators, not for external standards or end-users.
• Documentation improvement

PR checklist

• I have checked there is no other PR open for the same change.
• I have read the Contribution Guidelines.
• I grant the project the right to include and distribute the code under the BSD-3-Clause license (and I have the right to grant these rights).
• I have added tests to cover my changes.
• I have verified that the code complies with the projects coding standards.
• [Required for new sniffs] I have added XML documentation for the sniff.
• I have opened a sister-PR in the documentation repository to update the Wiki.

Changes are already covered by Squiz/Tests/Functions/FunctionDeclarationArgumentSpacingUnitTest.1.inc

[jrfnl]

@ntdiary Thanks for this PR! All good 👍🏻

Edit: for the record - I've checked and this sniff is not extended, so these changes do not affect any other sniffs.