The PR goes in the wrong direction.

The purpose of my change was to introduce new icon names ('folding-expanded','folding-collapsed') so that these can be themed individually by a product icon theme.
chevron-down/right are used at many places for various uses cases, not practical for themeing unless it's going too look like a chevron again.

So keep

const foldingExpandedIcon = registerIcon('folding-expanded', Codicon.chevronDown);
const foldingCollapsedIcon = registerIcon('folding-collapsed', Codicon.chevronRight);

and use foldingExpandedIcon.classNames or foldingExpandedIcon.cssSelector for wherever its class names are needed.