feat(terminal): add mechanism to fetch last focused terminal

[akinsho]

This is useful for plugins like flatten.nvim where you (really I) want to be able to check if the last toggled terminal was a float and if not not bother re-opening it e.g.

  callbacks = {
    pre_open = function() require('toggleterm').toggle() end,
    post_open = function(_, winnr)
      local term = require('toggleterm.terminal').get_last_focused() -- <--- Here
      if not term or not term:is_float() then term:toggle() end
      api.nvim_set_current_win(winnr)
    end,
    block_end = function() require('toggleterm').toggle() end,
  },