fix(extui): setup buffers after activating eventignorewin

[przepompownia]

Problem: setting up extui buffers sends redundant events.

require('vim._extui').enable({enable = true, msg = {target = 'msg'}})
vim.api.nvim_create_autocmd('BufFilePost', {
  callback = function (ev) print(ev.match) end
})

For example, gitsigns attempts to attach these buffers.

Solution: set them up when eventignorewin is active.

[przepompownia]

Cc @luukvbaal, @lewis6991 (side effect of lewis6991/gitsigns.nvim#1409 (comment))

[justinmk]

waiting for luukvbaal approval

[luukvbaal]

Thanks, I changed the order some more and added a clarifying comment.

[przepompownia]

Thanks for improvements @luukvbaal - I missed the autocommands related to window options here.