feat(coding-standard): add ignore_warnings flag

[damienwebdev]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/mage-os/github-actions/blob/main/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• Other... Please describe:

What is the current behavior?

By default, phpcs exits with a non-zero exit code when it finds warnings. Additionally, we fixed an issue with older versions of composer.

Fixes: #130
Supersedes: #132

What is the new behavior?

• We expose a new input called ignore_warnings that causes them to be reported, but still exit 0
• We both test against and fix an issue with older versions of composers that don't have plugins.

Does this PR introduce a breaking change?

• Yes
• No

Other information

[damienwebdev]

Awaiting #146 before merge!