Greetings,

It is my first time trying to contribute to Emacs source code.

The change is adding support for enabling stylistic set (font features) when using harfbuzz font backend. It is a commonly supported feature in the editors. See following links

The change adds :font-feature configuration like following

(set-face-attribute 'default nil :font "JetBrains Mono" :height 80
  :font-features '((zero . 1) (ss19 . 0) (calt . 1)))

The render result can be confirmed by adding "0" to composition table 

(set-char-table-range composition-function-table
  ?0
  '(["." 0 font-shape-gstring]))

I have read the CONTRIBUTE file and tried to make the commit message as clear as possible. Please let me know the code review process to get the patch accepted.