Fixes #16436. Allow filtering of indexable creation.

[NielsdeBlaauw]

Summary

This changeset standardises the order in which the type of the indexable page is determined. It also adds two filters, so developers can manipulate the order and logic with which the indexables are created far easier than is currently the case.

This PR can be summarized in the following changelog entry: changelog:bugfix, changelog: enhancement

• Fixes a bug where no SEO title would be found when a user visits a post type archive filtered by a taxonomy term.
• Adds two filters: wpseo_frontend_page_type_mapping and wpseo_frontend_indexable_mapping. These filters allow developers to change the order and logic of the creation of indexables.

Relevant technical choices:

• I've limited the changes to the methods that already exist with this logic. It seems overkill to seperate these checks out into different files.
• I've normalised the order of the checks of both functions. This possibly fixes a similair issue when a custom search result page is used.
• I've prioritised the term_archive line over the post_type_archive line, as I think in most cases the term archive title will be more specific and valuable than the post archive title.
• static_posts_page seems to be missing from the Indexable_Repository::for_current_page method. It is unclear if this is intentional.

Test instructions

Test instructions for the acceptance test before the PR gets merged

This PR can be acceptance tested by following these steps:

• See the reproduction in Yoast Title in custom post archive-posttype.php is blank when add more query string #16436.

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.

Other environments

• This PR also affects Shopify. I have added a changelog entry starting with [shopify-seo] and I have added test instructions for Shopify.

Documentation

• I have written documentation for this change.

Quality assurance

• I have tested this code to the best of my abilities
• I have added unittests to verify the code works as intended
• If any part of the code is behind a feature flag, my test instructions also cover cases where the feature flag is switched off.

Fixes #16436

[nlemoine]

Thanks @NielsdeBlaauw! I would love to see this kind of filters happen. It's now quite complicated to make WP SEO behave right when not fitting in one of the contexts already defined.

[enricobattocchi]

@NielsdeBlaauw sorry for the delay! We'll try to have a look in the upcoming weeks, we're working on a major improvements of our indexables creation and we need to check carefully how this could interact. I hope we can move it further soon.