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.

lua/nvim-tree/config.lua

@@ -24,12 +24,6 @@ function M.get_icon_state()
       symlink = "",
       symlink_open = "",
     },
-    lsp = {
-      hint = "",
-      info = "",
-      warning = "",
-      error = "",
-    },
   }
 
   local user_icons = vim.g.nvim_tree_icons