Add validation for et-al-min and et-al-use-first

[georgd]

Description

I accidently closed #415

This adds schema validation rule for et-al-min and et-al-use-first (see discussion in citation-style-language/documentation#141)

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• Variable addition (adds a new variable or type string)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist

• I have installed the repo pre-commit hook; if not, and I have modified any of the schema files, I have run trang and/or prettier on the files, per CONTRIBUTING
• I have performed a self-review of my own code
• I have included suggested corresponding changes to the documentation (if relevant)