Fix post tag duplication in ES query

[oscarssanchez]

Description of the Change

This PR aims to fix three things. First, there's a duplication of terms.post_tag.slug similar to the problem that was happening in #2242 when you are making with similar args:

$args = [
	'ep_integrate' => true,
	'tag' => array( 'slug1, slug2' ),
];

This is fixed here: https://github.com/10up/ElasticPress/compare/fix/post-tag-duplication?expand=1#diff-7004c1b9f26ece8d68d4d3b7b822331a1f1e0cdf34a5eda1aa146d930d21ad08R1007 adding post_tag and category as these are WP Core default taxonomies, and I have added the filter 'ep_post_tax_excluded_wp_query_root_check' in case this is needed to be extended by a developer.

Second, doing a query for more than one term in an array form does not work, as it will format the tag arg like a comma separated string.

Third, searching for multiple tags with comma separated tags does not work at all. This is allowed by core: https://developer.wordpress.org/reference/classes/wp_query/#tag-parameters

It looks like core will translate both the array and comma separated strings as comma separated, which is different than how it formats a tax query, therefore that part remains unaffected

Alternate Designs

Benefits

Expand WP_Query

Possible Drawbacks

Verification Process

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests passed.

Applicable Issues

Changelog Entry