[Feature Request] Different highlight group for opened and unopened folder icons

[KritPaoIn]

What I have right now:

What I would like to get:

[alex-courtis]

NvimTreeOpenedFolderName currently changes the opened folder text. NvimTreeFolderIcon applies to both open and closed icons.

Add NvimTreeFolderIconOpen, defaulting to NvimTreeFolderIcon.