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" }

[echasnovski]

This feature merged will make life of contributors and users a lot easier. For at least the following reasons:

• Linking allows specifying only the highlight group name without the need to search in which table the similar highlight group is previously defined.
• Linked highlight group can be safely defined before the group it is linked to.
• Linked highlight groups means that they should be highlighted similarly. This allows users to tweak only one (original) highlight group and the others will also change how they are displayed.