css variable support

I am getting this error:

```
Warning: Failed prop type: Prop style passed to SplitPane. Has invalid keys --left-resize-visible, --right-resize-visible
```

Because I am using CSS variables in the style field on a module. CSS variables are valid CSS rules and shouldn't throw any sort of warning. They always start with double dashes so that could be a simple v1 by just ignoring any key starting with double dashes.

Start at the style prop validation that emits the warning for SplitPane and inspect how property names are checked. Verify that CSS custom properties such as --left-resize-visible and --right-resize-visible no longer trigger warnings while other invalid keys remain rejected.

Written by the indexing model from the issue text.