Fix syntax highlighting for numeric separators

Screenshot from <https://v8.dev/blog/v8-release-75#numeric-separators>:

![](https://user-images.githubusercontent.com/81942/57871547-eed72400-77df-11e9-9ccd-cde8ce4b32f8.png)

- Upstream issue: https://github.com/PrismJS/prism/issues/1893
- Upstream patch by @RunDevelopment: https://github.com/PrismJS/prism/pull/1895
