Add epAutosuggestQueryFilter to allow filtering ES query based on input element

[johnwatkins0]

Description of the Change

Currently there is no easy way to filter an autosuggest query based on context. So in the case where you have a global autosuggest input on a page, there's no way to place a secondary autosuggest search on the same screen that covers, e.g., posts with a certain taxonomy term.

This follows the model of the existing filters, epDataFilter and epAutosuggestItemHTMLFilter to allow the ElasticSearch query to be modified before it is sent. We check if epAutosuggestQueryFilter is defined, and if it is, the query is passed through that function. The function also receives the searchText and the input element, the latter of which is key to customizing the query, as we can identify an input by its attributes or even access its form at input.form.

Closes #2908

Note: I didn't look into using @wordpress/hooks because my assumption is that package would bring in unwanted dependencies. If this option is pursued in the future, though, this functionality could be easily adapted. In that scenario I would recommend breaking each of these filters out into helper functions that can keep the current pattern for backward compatibility and fall back to the @wordpress/hooks pattern if the global function isn't defined.

How to test the Change

1. Checkout this branch and run npm run build or npm run watch
2. Go to a page with autosuggest
3. Enter the following in the JS console

window.epAutosuggestQueryFilter = (query, searchText, input) => {
    console.log({query, searchText, input});
    return query;
}

4. Type in the autosuggest input to see your callback running.

Changelog Entry

Added - Autosuggest: filter the autosuggest ElasticSearch query by defining a window.epAutosuggestQueryFilter() function in JavaScript.

Credits

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.