Update _ResizeHandle.pcss (#10772)

* Nesting: `mx_ResizeHandle`

* Nesting: `> div`

* Run prettier

* Use a custom property

* Remove a redundant declaration: `cursor: row-resize`

The resizer is either vertical or horizontal, and since `cursor: row-resize` is applied by default, it does not need to be repeated here.

* Conform the class names to the naming policy

* Revert "Use a custom property"

This reverts commit 6116939eec.

src/components/structures/MainSplit.tsx

@@ -87,7 +87,7 @@ export default class MainSplit extends React.Component<IProps> {
                     onResize={this.onResize}
                     onResizeStop={this.onResizeStop}
                     className="mx_RightPanel_ResizeWrapper"
-                    handleClasses={{ left: "mx_ResizeHandle_horizontal" }}
+                    handleClasses={{ left: "mx_ResizeHandle--horizontal" }}
                 >
                     {panelView}
                 </Resizable>