Fix corrupted library path of lua_ls

The previous code accidentally overwrote the library path of lua_ls
because it used `vim.tbl_extend` instead of `vim.list_extend`.

Because they paths that were added to the list (`luv` and `busted`) are
not really relevant for neovim configs, they are omitted entirely.

fixes: #2138

Author: oriori1703

init.lua

@@ -725,10 +725,7 @@ do
             checkThirdParty = false,
             -- NOTE: this is a lot slower and will cause issues when working on your own configuration.
             --  See https://github.com/neovim/nvim-lspconfig/issues/3189
-            library = vim.tbl_extend('force', vim.api.nvim_get_runtime_file('', true), {
-              '${3rd}/luv/library',
-              '${3rd}/busted/library',
-            }),
+            library = vim.api.nvim_get_runtime_file('', true),
           },
         })
       end,