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>

doc/nvim-tree-lua.txt

@@ -793,6 +793,7 @@ Use nvim-tree in a floating window.
 
 Highlight precedence, additive:
   git < opened < modified < bookmarked < diagnostics < copied < cut
+Neovim <= 0.8 will only show the highest.
 
 *nvim-tree.renderer.add_trailing*
 Appends a trailing slash to folder names.