opentype language tags for core text renderer

apparently the Core text renderer supports activating language tags in advanced opentype fonts, https://en.wikipedia.org/wiki/Apple_Advanced_Typography

the iosevka font implements fancy ligatures on a per-programming-language basis using these language tags.
https://github.com/be5invis/Iosevka/issues/56
(see also the README of this repo for examples)
the atom editor supports this via the use of custom stylesheets afaiu.

i find this a saner solution than enabling a fixed set of ligatures globally for a gui font, since the set of ligatures that make sense depends on the programming language. for example, is <= an arrow or a comparison operator? does |> have a special meaning in the language? etc.

so, feature request: could MacVim allow setting language tags for the Core Text renderer in opentype gui fonts? ideally this would be a new setting that can be put into a filetype-specific autocommand.

Start by locating MacVim's Core Text renderer and the GUI font-setting entry points mentioned by the request. Trace how a filetype-specific autocommand could provide OpenType language tags, and confirm the expected behavior with the Iosevka examples and linked references. Done means a documented setting can apply language tags per file type in Core Text fonts.