feat: support custom highlight for Tree Groups

[sudo-tee]

Currently there is almost no visual distinction between Groups and directory in the explorer view. This makes it difficult so seperate the Staged changes from the Changes as they are mapped to the Directory highlight.

It would be nice to have a new highlight for ExplorerGroup.

Here is an example of a custom Highlight

Without highlight

With Highlight

Thanks again for your great plugin

[sudo-tee]

If you want I can open a PR for this feature