fix(#2415): nvim 0.8 highlight overhaul support, limited to only show highest highlight precedence (#2642)

* fix: Add support for get_hl_defs in nvim 0.8

nvim-tree is using `nvim_get_hl` which was introduced in nvim 0.9 to
replace the unstable `get_hl_defs` in the following [commit](https://github.com/neovim/neovim/pull/22693/files).

Unfortunately this raises an error in 0.8 nvim versions due to the
function not existing.

```
Failed to run `config` for nvim-tree.lua
...are/nvim/lazy/nvim-tree.lua/lua/nvim-tree/appearance.lua:199: attempt to call field 'nvim_get_hl' (a nil value)
stacktrace:
  - ~/.config/nvim/lua/confidenceman02/plugins/nvim-tree.lua:14 _in_ **config**
  - ~/.config/nvim/lua/confidenceman02/lazy.lua:14
```

- Fall back to get_hl_defs when detecting 0.8
- Set the 'link' property to nil to emulate `link = false` in
  `builder.lua`

* fix(#2415): nvim 0.8 highlight overhaul support, limited to only show highest highlight precedence

---------

Co-authored-by: Jaime Terreu <jaime@terreu.com>
Co-authored-by: Alexander Courtis <alex@courtis.org>

lua/nvim-tree/renderer/builder.lua

@@ -295,16 +295,24 @@ function Builder:add_highlights(node)
     table.insert(name_groups, name)
   end
 
-  -- one or many icon groups
+  -- one or many icon groups; <= 0.8 always uses highest due to lack of a practical nvim_get_hl equivalent
   if #icon_groups > 1 then
-    icon_hl_group = self:create_combined_group(icon_groups)
+    if vim.fn.has "nvim-0.9" == 1 then
+      icon_hl_group = self:create_combined_group(icon_groups)
+    else
+      icon_hl_group = icon_groups[#icon_groups]
+    end
   else
     icon_hl_group = icon_groups[1]
   end
 
-  -- one or many name groups
+  -- one or many name groups; <= 0.8 always uses highest due to lack of a practical nvim_get_hl equivalent
   if #name_groups > 1 then
-    name_hl_group = self:create_combined_group(name_groups)
+    if vim.fn.has "nvim-0.9" == 1 then
+      name_hl_group = self:create_combined_group(name_groups)
+    else
+      name_hl_group = name_groups[#name_groups]
+    end
   else
     name_hl_group = name_groups[1]
   end