Add support for local overrides in lua/local.lua

GUI's like Neovide require system specific settings for font size based
on monitor resolution, pixel density, and scaling. Add support for
loading `lua/local.lua` if it exists to enable overriding the default
for `vim.o.guifont` or similar options.

.gitignore

@@ -1,2 +1,3 @@
 lazy-lock.json
 spell/*.spl
+lua/local.lua