fix: use correct codicon names for folding control colour customisation

[robertrossmann]

This fixes the editorGutter.foldingControlForeground colour contribution point by using the correct codicon names in the CSS.

Not sure when this regressed, I just noticed this while playing around with colour contributions and did not see any effect when I customised it.

[aeschli]

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.

[robertrossmann]

Thanks @aeschli for explanation - I pushed an updated version following your suggested approach. ✅

[aeschli]

Perfect, thanks!