Cache FileNode Icon And Highlight

HighlightedString for :highlighted_icon :highlighted_name should not change unless:

• options change
• name changes

Options:

1. Cache these in the Node (preferred)
2. add a cache around get_icon (will grow large, avoid)

1 is achievable for files as:

• It doesn't appear that name is set anywhere, hence safe to calculate once during construction.
• Executable is set once on construction/reload
• Explorer will be destroyed on config change, clearing cached values

1 is not achievable for directories as the icon changes based on whether the directory is open or grouped.

Plan:

• Add private members to FileNode that stores the nvim_tree.api.HighlightedString
  • hl_icon
  • hl_name
• FileNode and FileLinkNode populate members in the constructor, moving logic from:
  • :highlighted_icon
  • :highlighted_name

Start by locating the FileNode and FileLinkNode constructors and the highlighted_icon and highlighted_name implementations. Trace how highlighted values are currently produced for files and directories, then verify the intended cache lifetime against option changes, name changes, executable reloads, and directory state. Done means file values are reused while directory highlighting remains dynamic.