Add filterable method for getting the individual term HTML to the Facet widget.

[JakePT]

Description of the Change

Moves the markup for individual facet terms in the Facet widget into its own method, and adds a ep_facet_widget_term_html filter to the output to support filtering the HTML of individual facet terms.

Benefits

Allows filtering the HTML of individual facet terms without needing to filter the entire output of the widget.

Possible Drawbacks

None that I'm aware of, apart from the usual considerations when introducing filters.

Verification Process

• On a site with ElasticPress installed and content indexed, add the Facet widget and confirm that it continues to function as expected.
• Use the ep_facet_widget_term_html filter to change the HTML of individual terms and confirm that changes are reflected on the front end.

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

Addresses #2358.

Changelog Entry

Adds ep_facet_widget_term_html filter to the Facet widget for filtering the HTML of individual facet terms.

[Rahmon]

I'm wondering if it's worth adding a filter to $link... 🤔