support "Feature for Feature Variation" custom parameter

Glyphs.app has a "Feature for Feature Variation" to allow changing the feature tag used for GSUB FeatureVariations (bracket trick) from the default "rvrn" to a user-defined feature tag.

one problem with implementing support for this in glyphsLib is that

> designspace only has processing=[first|last], and uses rvrn and rclt respectively.
Unlike that Glyphs.app custom parameter, it doesn't let you specify an arbitrary feature tag.

So if user sets that `Feature For Feature Variation` to a tag that isn't either rvrn nor rclt, besides setting rules processing="last" inthe designspace file, we will have to also store that custom feature tag in the designspace.lib as a private key, and see how we can wire it up to ufo2ft and fontTools.varLib somehow.

Or maybe we just say we only support "rclt" as alternative non-rvrn feature tag for now, as that fits the designspace model and current library support.
And refine later.

Start by reviewing the designspace processing model and the existing handling of rvrn and rclt in glyphsLib. Then trace how designspace.lib data is consumed by ufo2ft and fontTools.varLib. Done requires a decided scope for arbitrary custom feature tags or a documented rclt-only limitation, with the relevant conversion path and tests updated.

Written by the indexing model from the issue text.