function addIconProvider(callback) {
        return FileTreeView.addIconProvider(callback);
    }
Adds an icon provider. The callback is invoked before each tree item is rendered, and can
return content to prepend to the item.

@param {!function(!{name:string, fullPath:string, isFile:boolean}):?string|jQuery|DOMNode|Preact.DOM.ins} callback
* `name`: the file or directory name
* `fullPath`: full path to the file or directory
* `isFile`: true if it's a file, false if it's a directory
Return a string of HTML text, a Preact.DOM.ins instance, a jQuery object, or a DOM node; or undefined
to prepend nothing.