chore: complete the diagnostic setup migration

Colors groups and icons are now in diagnostics.lua. They are defined on
setup which allows an easier configuration and better documentation.
`lsp_diagnostics` boolean value has been moved into a table `diagnostics`
with `enable` and `icons` as properties.

README.md

@@ -61,7 +61,15 @@ require'nvim-tree'.setup {
   -- updates the root directory of the tree on `DirChanged` (when your run `:cd` usually)
   update_cwd          = false,
   -- show lsp diagnostics in the signcolumn
-  lsp_diagnostics     = false,
+  diagnostics = {
+    enable = false,
+    icons = {
+      hint = "",
+      info = "",
+      warning = "",
+      error = "",
+    }
+  },
   -- update the focused file on `BufEnter`, un-collapses the folders recursively until it finds the file
   update_focused_file = {
     -- enables the feature
@@ -167,12 +175,6 @@ let g:nvim_tree_icons = {
     \   'empty_open': "",
     \   'symlink': "",
     \   'symlink_open': "",
-    \   },
-    \   'lsp': {
-    \     'hint': "",
-    \     'info': "",
-    \     'warning': "",
-    \     'error': "",
     \   }
     \ }