Add pronouns to user profile page and Person schema output.

[jdevalk]

Context

• In this pull, pronouns was added to Person schema's pending state. This pull implements it for Yoast SEO.

Summary

This PR can be summarized in the following changelog entry:

• Add a Pronouns field to the user profile and output these pronouns in the Schema.org output.

Relevant technical choices:

• Implemented as a Text value, as Schema.org states:

  We do not intend to enumerate all possible micro-syntaxes in all languages.

• Implemented both in admin/views/user-profile.php (which seems unused?) and in a new file src/user-meta/framework/custom-meta/author-pronouns.php.

Test instructions

Test instructions for the acceptance test before the PR gets merged

This PR can be acceptance tested by following these steps:

• Go to user profile page.
• Add pronouns.
• Check Schema Person output and see pronouns reflected in output.

Relevant test scenarios

• Changes should be tested on different posts/pages/taxonomies/custom post types/custom taxonomies
• Changes should be tested on multisite

Test instructions for QA when the code is in the RC

• QA should use the same steps as above.

UI changes

• This PR changes the UI in the plugin. I have added the 'UI change' label to this PR.

Documentation

• I have written documentation for this change. For example, comments in the Relevant technical choices, comments in the code, documentation on Confluence / shared Google Drive / Yoast developer portal, or other.

Quality assurance

• I have tested this code to the best of my abilities.
• During testing, I had activated all plugins that Yoast SEO provides integrations for.
• I have added unit tests to verify the code works as intended.
• I have checked that the base branch is correctly set.

[jdevalk]

Screenshots of this code in action: