Support alpha:desc option in sortFacetValuesBy

[balbesqueen]

Description
Allow sortFacetValuesBy to accept alpha:desc to enable reverse alphabetical sorting of facet values.

This is especially useful when facet values are chronological (e.g. years, versions), where it's desirable to show the most recent values first.

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Basic example
When indexing a list of media items or articles with a year facet, we currently get:

• 2020, 2021, 2022, 2023, 2024

But often we'd prefer to display:

• 2024, 2023, 2022, 2021, 2020

This could be achieved with:

{
  "sortFacetValuesBy": {
    "year": "alpha:desc"
  }
}