Ported over the maybe_change_filter_position() in Synonyms from the 10up ElasticPress codebase

[petenelson]

Description

The ep_synonyms_filter in the index settings is being added first before the lowercase filter, causing case-sensitive matching to applied to synonyms. Using the _settings endpoint on the ES index is showing this:

"default": {
	"filter": [
		"ep_synonyms_filter",
		"lowercase",
		"stop",
		"ewp_snowball"
	],

when it should be showing this

"default": {
	"filter": [
		"lowercase",
		"ep_synonyms_filter",
		"stop",
		"ewp_snowball"
	],

This has been fixed in the 10up ElasticPress codebase. https://github.com/10up/ElasticPress/blob/develop/includes/classes/Feature/Search/Synonyms.php#L895

Checklist

Please make sure the items below have been covered before requesting a review:

• This change works and has been tested locally (or has an appropriate fallback).
• This change works and has been tested on a Go sandbox.
• This change has relevant unit tests (if applicable).
• This change has relevant documentation additions / updates (if applicable).
• This change has the fix PRed upstream (if applicable). If not applicable, it has the relevant "// VIP: reason for the discrepancy with upstream" comment in places where the code is discrepant.

Steps to Test