Change autocomands for lua

[voyeg3r]

https://github.com/sunjon/Shade.nvim/blob/master/lua/shade.lua#L259

I think nvim already has autocomands, no need to keep old vimscript calls:

  -- setup autocommands -- TODO: set a precalculated winid
  api.nvim_exec([[
    augroup shade
    au!
    au WinEnter,VimEnter * call v:lua.require'shade'.autocmd('WinEnter',  win_getid())
    au WinClosed         * call v:lua.require'shade'.autocmd('WinClosed', expand('<afile>'))
    au TabEnter          * call v:lua.require'shade'.autocmd('TabEnter',  win_getid())
    au OptionSet         diff call v:lua.require'shade'.autocmd('OptionSet', win_getid())
    augroup END
  ]], false)