Add type hints to plugin options where possible

This could help beginners to get autocompletion, catch mistakes earlier,
and allow them to skip the docs for simple configs.

This is not perfect because a lot of the plugins type all of their keys
as required, even though they have defaults, but this is good enough.

lua/custom/plugins/init.lua

@@ -2,4 +2,7 @@
 --  I promise not to create any merge conflicts in this directory :)
 --
 -- See the kickstart.nvim README for more information
+
+---@module 'lazy'
+---@type LazySpec
 return {}