* feat(defaults.lua): add relative line numbers

* feat(keymaps.lua): add keymaps for buffer navigation, terminal, and nvimtree
* feat(nvim-tree.lua): add nvim-tree plugin with setup function

lua/custom/plugins/nvim-tree.lua

@@ -0,0 +1,12 @@
+return {
+    {
+        "nvim-tree/nvim-tree.lua",
+        version = "*",
+        dependencies = {
+            "nvim-tree/nvim-web-devicons",
+        },
+        config = function()
+            require("nvim-tree").setup {}
+        end,
+    }
+}