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`

Author: Jaime Terreu

Diff for: lua/nvim-tree/appearance.lua

@@ -196,8 +196,15 @@ function M.setup()
 
   -- hard link override when legacy only is present
   for from, to in pairs(LEGACY_LINKS) do
-    local hl_from = vim.api.nvim_get_hl(0, { name = from })
-    local hl_to = vim.api.nvim_get_hl(0, { name = to })
+    local hl_from
+    local hl_to
+    if vim.fn.has "nvim-0.8" == 1 then
+      hl_from = vim.api.nvim__get_hl_defs(0)[from] or {}
+      hl_to = vim.api.nvim__get_hl_defs(0)[to] or {}
+    else
+      hl_from = vim.api.nvim_get_hl(0, { name = from })
+      hl_to = vim.api.nvim_get_hl(0, { name = to })
+    end
     if vim.tbl_isempty(hl_from) and not vim.tbl_isempty(hl_to) then
       vim.api.nvim_command("hi link " .. from .. " " .. to)
     end

Diff for: lua/nvim-tree/renderer/builder.lua

@@ -266,7 +266,13 @@ function Builder:create_combined_group(groups)
 
     -- build the highlight, overriding values
     for _, group in ipairs(groups) do
-      local hl = vim.api.nvim_get_hl(0, { name = group, link = false })
+      local hl
+      if vim.fn.has "nvim-0.8" == 1 then
+        hl = vim.api.nvim__get_hl_defs(0)[group] or {}
+        hl["link"] = nil
+      else
+        hl = vim.api.nvim_get_hl(0, { name = group, link = false })
+      end
       combined_hl = vim.tbl_extend("force", combined_hl, hl)
     end