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 6116939eec7d9e4220b89a638623e5876e143adf.

src/components/structures/LoggedInView.tsx

@@ -264,7 +264,7 @@ class LoggedInView extends React.Component<IProps, IState> {
         const resizer = new Resizer(this._resizeContainer.current, CollapseDistributor, collapseConfig);
         resizer.setClassNames({
             handle: "mx_ResizeHandle",
-            vertical: "mx_ResizeHandle_vertical",
+            vertical: "mx_ResizeHandle--vertical",
         });
         return resizer;
     }