Use lexical binding in dotfile template

Lexical binding is easier to reason about and it's generally recommended for new code. It's faster too.

Dynamic scoping can lead to tricky situations. Example: https://emacs.stackexchange.com/questions/10394/scope-in-lambda

This probably won't break existing code. Most people don't even know the default is dynamic binding.

core/templates/.spacemacs.template

@@ -1,3 +1,4 @@
+;; -*- lexical-binding: t -*-
 ;; -*- mode: emacs-lisp -*-
 ;; This file is loaded by Spacemacs at startup.
 ;; It must be stored in your home directory.