Move nvim config into nvim/ subdirectory

Prepare repo structure for storing multiple dotfile configs.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

nvim/lua/plugins/treesitter.lua

@@ -0,0 +1,5 @@
+return {
+  "nvim-treesitter/nvim-treesitter",
+  build = ":TSUpdate",
+  lazy = false,
+}