Share we add link option for custom highlight?

[YuCao16]

This PR introduces the ability to link one highlight group to another within the plugin. This feature allows users to easily set up and manage complex highlight configurations.

[navarasu]

@YuCao16 Can you add an example to the doc?

[YuCao16]

@YuCao16 Can you add an example to the doc?

Sure, I will, later today.

[YuCao16]

@navarasu I have added one example in section Customization. By the way, I'd recommend we link NeoTreeWinSeparator to VertSplit:

NeoTreeWinSeparator = { link = "VertSplit" }