Feature: support custom folder icons in file-tree

Prerequisites

• I have searched for duplicate or closed feature requests
• I have read the contributing guidelines

Proposal

The ability to have custom folder icons the same way we can modify file icons in the file-tree. Including the ability to have custom folder icons based on folder names, as well as state between "open" and "closed" folders since some icon packs support different designs based on such.

Motivation and context

Many projects of mine which have used the file tree have custom file icons, and I've found that the default chevron style folder/directory doesn't match the style and design of the file tree very well at all.

To provide an example:

I'm using material icons here, and it would look great if the chevrons were indevidual folder icons instead.

Here's an example if it did: (from better-hub.com)