Move Nix files into a nix directory

Move everything from `config` to the root level.

nvim/plugin/comment.lua

@@ -0,0 +1,19 @@
+local status_ok, comment = pcall(require, "Comment")
+if not status_ok then
+  return
+end
+
+comment.setup {
+  padding = true,
+
+  opleader = {
+    line = "gc",
+    block = "gb",
+  },
+
+  mappings = {
+    basic = true,
+    extra = true,
+    extended = false,
+  },
+}