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/kickstart/plugins/gitsigns.lua

@@ -2,9 +2,14 @@
 -- NOTE: gitsigns is already included in init.lua but contains only the base
 -- config. This will add also the recommended keymaps.
 
+---@module 'lazy'
+---@type LazySpec
 return {
   {
     'lewis6991/gitsigns.nvim',
+    ---@module 'gitsigns'
+    ---@type Gitsigns.Config
+    ---@diagnostic disable-next-line: missing-fields
     opts = {
       on_attach = function(bufnr)
         local gitsigns = require 'gitsigns'