1914: php8 compat

[benlk]

Changes

This pull request makes the following changes:

• adds composer for phpunit
• adds instructions for running tests in docs/readme.md
• edits tests to support PHPUnit 9
• edits tests to support PHPUnit 10
• fixes issues identified by tests in PHP 7.4 and PHP 8
• adds phpcs rules for backwards compatibility
• adds docs for running phpcs
• runs phpcs
• remedies PHPCS backwards-compatibility tests
• changelog.md
• test with Co-Authors Plus

Why

For #1914

Testing/Questions

Features that this PR affects:

Questions that need to be answered before merging:

• Is this PR targeting the correct branch in this repository?
• Has the changelog.md file been updated to reflect the updates in this PR?
• [ ]

Steps to test this PR:

1. Run Largo on a PHP 8 environment
2. See tests/readme.md

Additional information

WP Buddy Client requesting: (if applicable)

• Contributor has read WP Buddy's GitHub code of conduct
• Contributor would like to be mentioned in the release notes as: Ben Keith
• Contributor agrees to the license terms of this repository.