Feat: add filetree plugin

- use neotree as the filetree plugin
- change keymap for float diagnostic messages

lua/custom/plugins/filetree.lua

@@ -0,0 +1,21 @@
+vim.cmd([[ let g:neo_tree_remove_legacy_commands = 1 ]])
+
+return {
+  "nvim-neo-tree/neo-tree.nvim",
+  version = "*",
+  dependencies = {
+    "nvim-lua/plenary.nvim",
+    "nvim-tree/nvim-web-devicons", -- not strictly required, but recommended
+    "MunifTanjim/nui.nvim",
+  },
+  cmd = "NeoTree",
+  keys = {
+    {
+      "<leader>e",
+      function()
+        require("neo-tree.command").execute({ toggle = true, dir = vim.loop.cwd() })
+      end,
+      desc = "Explorer NeoTree (cwd)"
+    }
+  }
+}