[Feature Request] Support Vim Sessions

If you use Vim Sessions, e.g. `:mksession Session.vim` and then `nvim -S Session.vim`, the terminal buffer is restored but toggleterm.nvim doesn't recognize the restored terminal buffer as a "toggleterm" buffer. So the next `:ToggleTerm` call creates a second terminal. There's also other issues as well, for example the restored terminal won't have tte same styling as if it were created using `:ToggleTerm`.

Being able to restore the toggleterm terminal will likely require https://github.com/akinsho/toggleterm.nvim/issues/528 so I'd consider the two issues to be related.