No distinct highlight group for group headers in explorer

[thefapinstructor]

Bug

The group headers in the explorer (e.g. Changes (18), Staged Changes (77)) use the Directory highlight group, which is the same group used for folder names. This makes them visually indistinguishable from directories.

Fix

Introduce a CodeDiffGroupHeader highlight group and use it for group-type nodes in prepare_node in lua/codediff/ui/explorer/nodes.lua:

if data.type == "group" then
  line:append(" ", "CodeDiffGroupHeader")
  line:append(node.text, "CodeDiffGroupHeader")

And define the highlight in lua/codediff/ui/highlights.lua:

vim.api.nvim_set_hl(0, "CodeDiffGroupHeader", { link = "Title", default = true })

This allows users to customize the group header appearance independently from folder names.